Corpus was originally a Latin word. Its basic meaning is 'body'. The plural of corpus is corpora: see -us in Latin. Although some people say corpuses, this is frowned on - at least by academics who like to pretend that they speak Latin.
Outside Law and Anatomy, where it is usually part of a Latin phrase (e.g. corpus delicti and corpus callosum respectively), in modern English corpus is mostly used to mean 'a body, or collection, of writing or texts'. We can talk of 'the corpus of the Civil Law', meaning all the written basis of the Civil Law, or 'the corpus of Shakespeare's writings', meaning the Complete Works; and dictionaries are increasingly based on corpora of texts collected from language in use.
Corpus belonged to a smallish set of Latin nouns ending in -us that formed plurals in -ora. (Most nouns that ended in -us formed plurals in -i.)