The acronym stands for “Comprehensive TeX Archive Network”, which more-or-less specifies what it's for:
The basic framework was developed by a TUG working group set up to resolve the (then existing) requirement for users to know on which archive site a particular package might be found.
Actual implementation offers three distinct types of host:
Note that there is nothing to prevent any archive from supporting other functions, so a CTAN mirror may also operate as a CPAN (Perl) mirror and as a SourceForge (general free software) mirror, and …
Functions carried out by the core archive are:
Users may make direct contact with the CTAN management team.
Users should ordinarily download material from CTAN via the archive selector: this uses the mirror monitor's database, and uses the caller's geographical location to offer an efficient choice of “sufficiently up-to-date” mirror site for you to connect to. This procedure has the advantage of distributing the load on CTAN mirrors.