The concept of carrying out research on written or spoken texts is not restricted to corpus linguistics. Indeed, individual texts are often used for many kinds of literary and linguistic analysis - the stylistic analysis of a poem, or a conversation analysis of a tv talk show. However, the notion of a corpus as the basis for a form of empirical linguistics is different from the examination of single texts in several fundamental ways.
In principle, any collection of more than one text can be called a corpus, (corpus being Latin for "body", hence a corpus is any body of text). But the term "corpus" when used in the context of modern linguistics tends most frequently to have more specific connotations than this simple definition. The following list describes the four main characteristics of the modern corpus.
Sampling and representativeness
Finite size
Machine-readable form
A standard reference

Often in linguistics we are not merely interested in an individual text or author, but a whole variety of language. In such cases we have two options for data collection:
We could analyse every single utterance in that variety - however, this option is impracticable except in a few cases, for example with a dead language which only has a few texts. Usually, however, analysing every utterance would be an unending and impossible task.
We could construct a smaller sample of that variety. This is a more realistic option.