It has been more than five years passed since saurik launch the Cydia on year 2009 and the jailbreak community grows each day. We can say that cydia is the intermediate or bridge that connects end-users with repositories. Currently only working and available way you could get cydia is via a jailbreak tool like redsn0w, greenpois0n, evasi0n, absinthe and other jailbreak tools and during the time of jailbreaking process you need to opt out for cydia installation in order to download cydia .deb version installed on your Apple device like iPhone, iPod touch and iPad.
Earlier when I heard about jailbreak process, I was thinking that it might be illegal as we're just allowing our device to install third party apps also you may see the warning message saying that after downloading cydia you're Apple device warranty will be void, But I think Its worth to use this wonderful and helpful software. Since Apple's apps store is limited and most of the apps are paid only, But when you jailbreak and install cydia you'll have endless control over your device to add N numbers of themes, apps, icons, wallpapers from different repositories and other third part owners.
Most shouted reason to install cydia on your Apple device is that it comes with so many interesting tweaks including Activator, vShare, Installous and many more. Using of some these tweaks you can enjoy functions like Wi-Fi, Bluetooth or do any other function if an app supports it. Another most appealing tweak force you to jailbreak is WinterBoard. Its giant and most downloaded tweaks by Cydia users. It allow you to customize your device using various available themes.
According to www.CydiaRepo.com, there are so many interesting things you can do with your iDevice after installing cydia including themes, icon setting, tweaks, game emulators, apps, ringtones and many more that you can think of. By default it comes with few highly important repo like BigBoss, Modmyi etc but you can also add another repositories to take advantage of these nice features.
So what have you decided? Do you want to get Cydia on your iDevice or still thinking to use ordinary Apple stores.