What is a Concordancer?
A concordancer is like a search engine that can be used for studying language (corpus linguistics). You simply enter a word or phrase that you want to know about and search in the normal way. Concordancers are a tools for corpus linguistics. Since most corpora (bodies of text) are incredibly large, it is a fruitless enterprise to search a corpus without the help of a computer.
What does a Concordancer do?
Concordance programs turn the electronic texts into databases which can be searched. Usually word queries are always possible, but most programs also offer the possibility of searching for word combinations within a specified range of words and of looking up parts of words (substrings, in particular affixes, for example). If the program is a bit more sophisticated, it might also provide its user with lists of collocates or frequency lists.
The Concordance or Results
The results are presented in a different way: instead of giving you a list of files or websites containing the search word or phrase, you'll get a list of phrases or sentences with the search word or phrase centred. This allows you to look for patterns, such as whether the word usually or frequently comes at the beginning of sentence or whether it is followed by certain words, like prepositions.
As an example, a search for 'involved' would quickly show that is often used with 'in' and 'with'. A closer examination of the examples would help you to find the reason why.
What do Concordancers search?
A Concordancer searches what is called a corpus (the plural is corpora), which is basically a database of language. There are many in existence, both general and specialised, although no corpus can be regarded as complete.
Do remember that concordancers are only as good as the database they are searching.