Cydia is just like apple appstore but it is for jailbroken devices which inlude iphone ipod or ipad, Cydia is not found or downloaded from the apple appstore and it is not a website in order to get you can install this by using “jailbreak tool” such as Pangu, Evasion or Redsnow.
If you want to install Cydia on your idevice you will have to jailbreak this. Suppose you are running on ios 7. 1. 2 then download the Pangu tool here:-
Now open cydia and wait for its preparation. After preparing Cydia will exit and again open it.
Tap add source
Now tap the search button and write down the name of app or tweak name you want
Suppose Barrel, it is most downloaded app by cydia users. It add graphical effect to your home screen
Now tap on the app or tweak you want, tap confirm and install it
(the name shown in blue words is paid and the same name shown in black words is free)
Hope you understand,for more help comment below