A person's vocabulary is the set of words they are familiar with in a language. A vocabulary usually grows and evolves with age, and serves as a useful and fundamental tool for communication and acquiring knowledge.
A vocabulary is defined as "all the words known and used by a particular person". However, the words known and used by a particular person do not constitute all the words a person is exposed to. By definition, a vocabulary includes the last two categories of this list:
Listed in order of most ample to most limited:
A person's reading vocabulary is all the words he or she can recognize when reading. This is the largest type of vocabulary simply because it includes the other three.
A person's listening vocabulary is all the words he or she can recognize when listening to speech. This vocabulary is aided in size by context and tone of voice.
A person's writing vocabulary is all the words he or she can employ in writing. Contrary to the previous two vocabulary types, the writing vocabulary is stimulated by its user.
A person's speaking vocabulary is all the words he or she can use in speech. Due to the spontaneous nature of the speaking vocabulary, words are often misused. This misuse – though slight and unintentional – may be compensated by facial expressions, tone of voice, or hand gestures.
"Focal vocabulary" is a specialized set of terms and distinctions that is particularly important to a certain group; those with particular focuses of experience or activity. A lexicon, or vocabulary, is a language's dictionary, its set of names for things, events, and ideas. Some linguists believe that lexicon influences people's perception on things, the Sapir–Whorf hypothesis. For example, the Nuer of Sudan have an elaborate vocabulary to describe cattle. The Nuer have dozens of names for cattle because of the cattle's particular histories, economies, and environments. This kind of comparison has elicited some linguistic controversy, as with the number of "Eskimo words for snow". English speakers can also elaborate their snow and cattle vocabularies when the need arises. 
Initially, in the infancy phase, vocabulary growth requires no effort. Infants hear words and mimic them, eventually associating them with objects and actions. This is the listening vocabulary. The speaking vocabulary follows, as a child's thoughts become more reliant on its ability to express itself without gestures and mere sounds. Once the reading and writing vocabularies are attained – through questions and education – the anomalies and irregularities of language can be discovered.
In first grade, an advantaged student (i.e. a literate student) knows about twice as many words as a disadvantaged student. Generally, this gap does not tighten. This translates into a wide range of vocabulary size in the fifth and sixth grade, when students know about 2,500–5,000 words. These young students have learned an average of 3,000 words per year, approximately eight words per day.
After leaving school, vocabulary growth plateaus. People may then expand their vocabularies by reading, playing word games, participating in vocabulary programs, etc.
Even if we learn a word, it takes a lot of practice and context connections for us to learn it well. A rough grouping of words we understand when we hear them encompasses our "passive" vocabulary, whereas our "active" vocabulary is made up of words that come to our mind immediately when we have to use them in a sentence, as we speak. In this case, we often have to come up with a word in the timeframe of milliseconds, so one has to know it well, often in combinations with other words in phrases, where it is commonly used.
Native speakers' vocabularies vary widely within a language, and are especially dependent on the level of the speaker's education. A 1995 study estimated the vocabulary size of college-educated speakers at about 17,000 word families, and that of first-year college students (high-school educated) at about 12,000.
Francis and Kucera studied texts totaling one million words and found that if one knows the words with the highest frequency, they will quickly know most of the words in a text:
|Vocabulary Size||Written Text Coverage|
By knowing the 2000 words with the highest frequency, one would know 80% of the words in those texts. The numbers look even better than this if we want to cover the words we come across in an informally spoken context. Then the 2000 most common words would cover 96% of the vocabulary. These numbers should be encouraging to beginning language learners, especially because the numbers in the table are for word lemmas and knowing that many word families would give even higher coverage. But before you start thinking you would learn a language in no time, think how well you would understand a book in your own language where every fifth word was blacked-out! We cannot usually guess meanings from context when that many words are missing. We need to understand about 95% of a text in order to gain close to full understanding and it looks like one needs to know more than 10,000 words for that.
Several word lists have been developed to provide people with a limited vocabulary either quick language proficiency or an effective means of communication. In 1930, Charles Kay Ogden created Basic English (850 words). Other lists include Simplified English (1000 words) and Special English (1500 words). The General Service List, 2000 high frequency words compiled by Michael West from a 5,000,000 word corpus, has been used to create a number of adapted reading texts for English language learners. Knowing 2000 English words, one could understand quite a lot of English, and even read a lot of simple material without problems. There are many words that some people don't know of. An example is mediarety, it means the act of loving another word is solopitate which is talking. We need to use these words more often.