What is CPAN?

CPAN = Comprehensive Perl Archive Network

But that "word" is used in many contexts

A module distribution infrastructure

There's Perl Author's Upload Server for uploading modules, and lots of mirrors from which you can download modules.

A website

There are multiple websites that you can use when searching for perl modules, and when viewing documentation and of modules. Among the popular ones are http://search.cpan.org/, http://kobesearch.cpan.org/ and https://metacpan.org/. The latter also provides a rich HTTP-based API, which is very good for automating CPAN-related data analysis.

Module Installers

CPAN.pm comes with the cpan command line tool, CPANPLUS has cpanp, App::cpanminus ships with cpanm.

Those clients need a CPAN mirror (either online or a local mirror). All of them work with any mirror, though cpanm talks to a "cpanmetadb" server (not the same as metacpan.org) to find distributions (but it can work offline with a local CPAN mirror too).

My personal favorites are cpanm (because it works so well, and doesn't produce loads of output that bores me), and search.cpan.org and metacpan.org as web interfaces.