What is Cydia?
Cydia is an Installer for the iPhone and iTouch which is not exactly prepared by the Company Apple or neither it is an official installer for the iPhone, Cydia was created and is maintained bu Jay Saurik freeman ( known as 'Saurik' ).
Cydia can be used to download applications whicha re not available in the itunes store, thse applications are mostly free only a few being sold. (gues who will buy an unofficial thing without a gurantee).
Cydia is an item which is available during Jail breaking the iPhone using Quickpawn which released YellowsnOw and will release RedsnOw for Apples new iPhone OS 3.0.
How to download applications through Cydia ?
You have to just install Cydia onto your iPhone or iTouch and then it is very easy to understand how to install it unless you are a complete idiot. YOu can get a variety of applications for example the one which i like are regarding Video recording in the iPhone.
Cydia is a front-end graphical interface to two open source techs - APT and DPKG. DPKG is a system that installs and removes specially prepared packages. APT allows these files to be organized in network repositories to be indexed and downloaded.
I must say that you can get some awesome stuff using this if you searh on the internet which can make your iPhone and iTouch one to envy for.
Cydia Homepage by Saurik.