In international relations, public diplomacy or people's diplomacy, broadly speaking, is the communication with foreign publics to establish a dialogue designed to inform and influence. There is no one definition of Public Diplomacy, and may be easier described than easily defined as definitions have changed and continue to change over time. It is practiced through a variety of instruments and methods ranging from personal contact and media interviews to the Internet and educational exchanges.
Nicholas J. Cull of the USC Center on Public Diplomacy, wrote in his essay "'Public Diplomacy' Before Gullion: The Evolution of a Phrase:
"The earliest use of the phrase "public diplomacy" to surface is actually not American at all but in a leader piece from The Times in January 1856. It is used merely as a synonym for civility in a piece criticizing the posturing of President Franklin Pierce.The modern meaning of the term "public diplomacy" was coined in 1965 by Edmund Gullion, dean of the Fletcher School of Law and Diplomacy at Tufts University and a distinguished retired foreign service officer, when he established an Edward R. Murrow Center of Public Diplomacy
The brochure for the new center provided a description of the concept: “Public diplomacy… deals with the influence of public attitudes on the formation and execution of foreign policies. It encompasses dimensions of international relations beyond traditional diplomacy; the cultivation by governments of public opinion in other countries; the interaction of private groups and interests in one country with another; the reporting of foreign affairs and its impact on policy; communication between those whose job is communication, as diplomats and foreign correspondents; and the process of intercultural communications.” 
Over time, the concept and definition has evolved by various practitioners. Other definitions and descriptions include:
The most important roles public diplomacy will have to play for the United States in the current international environment will be less grand-strategic and more operational than during the Cold War. Support of national policy in military contingencies is one such role, and probably the most important.- Carnes Lord (former Deputy Director USIA), Professor of Statecraft and Civilization, October 1998 
Public diplomacy - effectively communicating with publics around the globe - to understand, value and even emulate America's vision and ideas; historically one of America's most effective weapons of outreach, persuasion and policy.- Jill A. Schuker (former Senior Director for Public Affairs at the National Security Council) July 2004
Public diplomacy may be defined, simply, as the conduct of international relations by governments through public communications media and through dealings with a wide range of nongovernmental entities (political parties, corporations, trade associations, labor unions, educational institutions, religious organizations, ethnic groups, and so on including influential individuals) for the purpose of influencing the politics and actions of other governments.- Alan K. Henrikson, Professor of Diplomatic History, April 2005.
Public diplomacy that traditionally represents actions of governments to influence overseas publics within the foreign policy process has expanded today - by accident and design - beyond the realm of governments to include the media, multinational corporations, NGO's and faith-based organizations as active participants in the field.Crocker Snow Jr., Acting Director Edward R. Murrow Center, May 2005.
"PUBLIC DIPLOMACY refers to government-sponsored programs intended to inform or influence public opinion in other countries; its chief instruments are publications, motion pictures, cultural exchanges, radio and television." (U.S. Department of State, Dictionary of International Relations Terms, 1987, p. 85)
The United States Information Agency (USIA), which was the main government agency in charge of Public Diplomacy until it merged with the Department of State in 1999, described it as:
Public diplomacy seeks to promote the national interest and the national security of the United States through understanding, informing, and influencing foreign publics and broadening dialogue between American citizens and institutions and their counterparts abroad.
For The Planning Group for Integration of USIA into the Dept. of State (June 20, 1997), public diplomacy is defined as follows: "Public Diplomacy seeks to promote the national interest of the United States through understanding, informing and influencing foreign audiences." 
According to Hans N. Tuch, author of Communicating With the World (St. Martin's Press, NY, 1990), public diplomacy is defined as: "Official government efforts to shape the communications environment overseas in which American foreign policy is played out, in order to reduce the degree to which misperceptions and misunderstandings complicate relations between the U.S. and other nations." 
Standard diplomacy might be described as the ways in which government leaders communicate with each other at the highest levels, the elite diplomacy we are all familiar with. Public diplomacy, by contrast focuses on the ways in which a country (or multi-lateral organization such as the United Nations) communicates with citizens in other societies. A country may be acting deliberately or inadvertently, and through both official and private individuals and institutions. Effective public diplomacy starts from the premise that dialogue, rather than a sales pitch, is often central to achieving the goals of foreign policy: public diplomacy must be seen as a two-way street. Furthermore, public diplomacy activities often present many differing views as represented by private American individuals and organizations in addition to official U.S. Government views.
Traditional diplomacy actively engages one government with another government. In traditional diplomacy, U.S. Embassy officials represent the U.S. Government in a host country primarily by maintaining relations and conducting official USG business with the officials of the host government whereas public diplomacy primarily engages many diverse non-government elements of a society.
Film, television, music, sports, video games and other social/cultural activities are seen by public diplomacy advocates as enormously important avenues for otherwise diverse citizens to understand each other and integral to the international cultural understanding, which they state is a key goal of modern public diplomacy strategy. It involves not only shaping the message(s) that a country wishes to present abroad, but also analyzing and understanding the ways that the message is interpreted by diverse societies and developing the tools of listening and conversation as well as the tools of persuasion.
One of the most successful initiatives which embodies the principles of effective public diplomacy is the creation by international treaty in the 1950s of the European Coal and Steel Community which later became the European Union. Its original purpose after World War II was to tie the economies of Europe together so much that war would be impossible. Supporters of European integration see it as having achieved both this goal and the extra benefit of catalysing greater international understanding as European countries did more business together and the ties among member states' citizens increased. Opponents of European integration are leery of a loss of national sovereignty and greater centralization of power.
Public diplomacy has been an essential element of American foreign policy for decades. It was an important tool in influencing public opinion during the Cold War with the former Soviet Union. Since the attacks of September 11, 2001, the term has come back into vogue as the United States government works to improve their reputation abroad, particularly in the Middle East and among those in the Islamic world. Numerous panels, including those sponsored by the Council on Foreign Relations, have evaluated American efforts in public diplomacy since 9/11 and have written reports recommending that the United States take various actions to improve the effectiveness of their public diplomacy.
The United States Advisory Commission on Public Diplomacy was established in the late 1940s to evaluate American public diplomacy effort. The Commission is a seven member bipartisan board whose members are nominated by the President and confirmed by the United States Senate. William Hybl is the current chair, and other members include former Ambassadors Lyndon Olson and Penne Percy Korth Peacock, as well as Jay Snyder, John E. Osborn and Lezlee Westine. In 2008, the Commission issued a report entitled "Getting the People Part RIght" that was largely critical of the State Department's personnel policies and practices in hiring, training and promoting public affairs and public diplomacy officers. The report attracted significant attention, and was the subject of a Senate hearing.
This traditional concept is expanded on with the idea of adopting what is called "population-centric foreign affairs" within which foreign populations assume a central component of foreign policy. Since people, not just states, are of global importance in a world where technology and migration increasingly face everyone, an entire new door of policy is opened.
Public Diplomacy is not to be confused with Citizen Diplomacy (or people's diplomacy), and Grassroot diplomacy: Citizen Diplomacy is a broad political concept referring to an activity whereby ordinary citizens represent their country either by design or inadvertently when abroad or engaging with people overseas. This is sometimes necessary when official avenues of diplomacy are unavailable. The champion of Citizen Diplomacy was Dr. Robert Fuller who travelled frequently to the Soviet Union during the Cold War. Grassroot Diplomacy is a recent political concept which describes the activity of individuals at grassroots level connecting with government officials to promote and implement policy-related projects. Pioneered by the Grassroot Diplomat Talyn Rahman-Figueroa , grassroot diplomacy refers to the process of "bridging the gap" between members of civil society and government-level decision members.
There are many methods and instruments that are used in Public Diplomacy. Nicholas Cull divides the practice into five elements: listening, advocacy, cultural diplomacy, exchange diplomacy and international broadcasting (IB). 
Methods such as personal contact, broadcasters such as the Voice of America, Radio Free Europe and Radio Liberty  exchange programs such as Fulbright and the International Visitor Leadership program, American arts and performances in foreign countries, and the use of the Internet are all instruments used for practicing Public Diplomacy depending on the audience to be communicated with and the message to be conveyed.