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.
Mirrors
There is a team in charge of mirrors.
A list of mirrors :
Distributions
Here is the list of the software included 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
How to create repository index files
How to set up a debian repository with a list of available tools.
How to set up a trivial archive
A Tutorial on Working with Repositories