You need to install CVS to use it in a command line.
The document "Eclipse Compatibility with CVSNT and CVS 1.12.x" mentions:
The Eclipse CVS client is a Java implementation of a CVS client that does not require any other CVS client to be installed.
The advantage of this is that Eclipse CVS can be used without requiring the user to install any additional software.
See also compatibility FAQ:
1/ Does Eclipse use [WinCVS|CVS command-line client] to talk to the server?
No. Eclipse implements a CVS client in Java that talks directly to the server using the documented CVS protocol. No external CVS client is required.
2/ The command-line CVS client stores information in CVS folders. Does Eclipse do the same thing? If so, where are the folders?
Eclipse stores CVS sync information in CVS/ folders in the same way as the command-line CVS client does.
However, you rarely see these folders within Eclipse. They are marked using a Core facility called "team-private" which causes them to be hidden from view.
If you open a (non-Eclipse) file explorer you will see that these directories and their contents appear on the file system.
3/ Is Eclipse compatible with the command-line CVS client?
Eclipse Team CVS stores its meta information in a format that is compatible with the command-line CVS client. Thus you should be able to use a CVS command line client against Eclipse workspace files on disk. Please note that this support is experimental, and you may run into problems.