If you are just learning a new language, your first goal is to take care of basic tasks like shopping, greeting others, and getting directions. Language books for tourists focus on words and phrases such as: "hello," "good-bye," "what time is it?," "where is the train station?" "how much is that?" and "that's too much."
For those who already know a language and wish to improve vocabulary, the direction of improvement depends on the intended use. Some jobs involve terms particular to the occupation, and anyone wishing to advance will want to learn the precise meaning of those terms. To generally improve vocabulary, it's good to focus on the kinds of words that turn up in texts and word lists for vocabulary development. Many of these words describe people, such as "autodidact," which refers to someone who is self-taught.
The best preparation for language tests, whether they call for reading, writing, listening or speaking, is to study words from lists for expanding vocabulary. Virtual Salt has compiled a list of 1062 words that are at just the right level to be helpful for test-takers. The list includes words like "benign," "covert" and "cull." These are not obscure or rare. They are words that some people might not know but that are part of the everyday vocabulary of those with better command of the language. Therefore, they are likely to show up in critical reading segments of tests. They are also words that enhance the impression of the test-taker's written responses. Knowing them will raise reading and writing scores.
According to Dr. Ken Barker of the University of Texas, Austin, the Longman Defining Vocabulary is made up of approximately 2,000 words that are used in writing definitions of other words. These words make up the Longman Dictionary of Contemporary English. Amazing as it may sound, the words in the Longman Dictionary can be used to talk about anything, as well as being used to define all other words.