cvs is a software tool that is often used when a group of people are all editing the same set of documents. It is often used when a group of people are working on a software project. The cvs software acts like a librarian. Only one person can "check out" a document for editing at any time. Other people can read a document that is checked out but cannot put an edited version back into the document pool (repository).
cvs can also be used for office documents or any group of documents that may be edited by a group of people. It is most often used for software development.
A cvs machine could be a file server that is using cvs or it could be a machine where software developers log in interactively and which uses cvs for files on the same machine.