There are tens of thousands of words in the English language, and along with social and scientific progress, new words appear frequently. The total number of English words, if it could be found out, must be surprisingly large. But we need not be worried about the impossibility of learning all of them, for only a few thousand words are used by ordinary people for ordinary purposes. These are the most useful words, or the common words（一般词汇）, words that form the core of the English vocabulary. They are the words we must learn and remember.

Apart from the common words, there are words used by people of special professions or fields, on special occasions or for special purposes. Among these are political, legal, scientific, technical, business and literary words. It would be good for us to know some very useful words in various fields, and many of the words related to the field we will work in or be associated with. These words are generally formal, and may be called formal and technical words（正式词汇和技术词汇）.

There is another type of words: those used by people who are not well edu-cated or by people of special groups, such as people of a particular region or an age group. Some of these words may not be understandable to people in general, and may disappear after a short period of time. Some of them may continue to be used, become acceptable to all people and join the common words. Among these words are slang, jargon, dialectal and obsolete words（俚语、行话、方言和废弃词）. They may be called nonstandard words（非标准词）. We need to understand them but should avoid using them, unless in special situations.