Change the design on lock screen on my iPhone?

I've seen on someone's iPhone before that when it's on the lock screen the font is all different? And the sliding bit where you unlock it is different too? Is it in the settings or an app because I really want to change mine but don't know how? I have the iPhone 4S if it helps?

Sounds like an app called cydia. But to get cydia you have to jailbrake your iPhone and download it from the Internet.

You can get different lockscreens from Cydia. But first you need to install Winterboard (Cydia), in order to use those lockscreens.