What are repositories?
Repositories (also called sources) host packages. You can think of Cydia a bit like a web browser: consider that Safari doesn't contain or own Google or Amazon, but instead is a way to view Google, Amazon, and any other website. Cydia is a way to browse and view software hosted by independent repositories, and the Cydia Store is a purchasing system that lets you buy software hosted by them.
Cydia automatically includes a few sources, but only Cydia/Telesphoreo is run by the people who make Cydia. The sources BigBoss, ModMyi, and ZodTTD/MacCiti are independently-owned and work closely with Cydia.
Cydia is an open platform built to help you have control over your device; you can choose to add extra repositories run by other people and install any package you want, from good to bad.
Why are there ads in Cydia?
Repositories choose to cover their costs (including the costs of hosting packages and having staff check package submissions) by displaying ads on their pages within Cydia. These ads are not controlled by Cydia itself; they are the responsibility of the repositories, and none of the money from the ads goes to Cydia.
How do I make a repository to host and share packages I made?
Here is the official guide to making your own Cydia repository: http://www.saurik.com/id/7
How do I ask one of the default repositories to host my package?
You can submit packages you have made to one of the default repositories (the sources pre-installed in Cydia) if you would like to make your packages available to nearly everyone who uses Cydia. Each default repository offers hosting of packages and provides services like realtime download statistics.
or email email@example.com
I uninstalled a default repository. How do I reinstall it?
Go to the Cydia homepage and tap More Package Sources. If you've uninstalled any of the default sources, they'll show up on that page so that you can reinstall them.