A Debian repository is a directory where you can find software you can install on Debian.
Each Debian distribution has its repository.
The Debian repositories can be found on Debian mirrors.
There is a team in charge of mirrors.
A list of mirrors :
Here is the list of the software include in each Debian distribution :
in the stable repository
in the lenny-backports repository
in the testing repository
in the sid repository
in the experimental repository
Working with repositories
There you can find a list of ?how-tos.