You need an Covert Ops Cloaking Device on Covops or Stealth Bombers, they're the best but only some ships can fit them. The others have speed penalties, meaning you go really slow when cloaked, and your sensors/locking ability doesn't work for a time after you uncloak (and I don't think you can warp with them, although I'm not certain on that, the speed penalty means you can't get up to warp speed.)
Basically, you hit a button and you cloak, turning you invisible to everyone. You can't cloak if you've been targetted succesfully. You can't cloak and will decloak if you come within 2000m of something.
The best way to get off gates is to hit your MWD, boosting your speed, then hit your cloak. That way you're moving away quickly and any chasers who are trying to decloak you will have a tough time catching you. Some people recommend changing direction once you cloak, to fool the decloakers more. So that means you wouldn't align when you're coming off the gate, but would pick a different direction to MWD to, and then align once you become invisible.
Cloaked ships are good for low-sec and 0.0, apart from getting off gates, they're handy for ratting and plexing. If you spot someone unfriendly coming into the system you cloak up until they leave, meaning they can't scan or probe you down. The problem with travelling in 0.0 though is bubbles. People have drag bubbles, that when you warp to a gate in line with them, they literally drag you 75km or so off the gate to the bubble. While this won't uncloak you (I believe) most people sitting on a bubble drop a jetcan at the edge of the bubble, so that will decloak you. Then you have to do your MWD and cloak trick to get to a position you can warp away.
But yes, cloaks are very valuable in low and null sec.