I asked this same question a few months ago. Take a look at the answers I got: http://www.italki.com/question/251713 When I first came across The Urban Dictionary, I was shocked and annoyed. It appeared to be full of vulgar nonsense. Then, after a while, I realised it's actually quite funny. What you have to realise is that it's not a 'real' dictionary. A lot of the phrases don't actually exist, or have been made up by a group of friends and posted there in UD just for fun. As it's mainly teenage boys who post there, it's not surprising that it's full of insults and vulgarity. Yes, you may find a definition of the slang word or phrase that you're looking for. I've used it myself occasionally. But please follow the advice which another member gave me - take everything you read in the UD 'with a pinch of salt'. Or in other words, DON'T believe everything see there.

I wouldn't look up words like "cat" in Urban dictionary, but it helped me to understand some slang from the American movies I've seen.

I wrote my opinion about it here: http://www.italki.com/question/276270 Like Su Ki said, most of the entries are jokes meant to be funny. For example, one of the definitions of 'cat' is "A quite pleasant furry creature that vaguely resembles a meatloaf. Cats are the most intellectually superior creature on Earth." I just looked up a few completely random, but common, words and almost every single entry was a joke, and the actual definition was often never even mentioned. A lot of entries are just meant to make the makers laugh, some are puns, some are insulting to whatever nationality, racial group, country or political party the maker happened to hate. Someone entered in the Chinglish phrase "No zuo no die". You can tell from the example sentences that it's making fun of Chinese people who can't speak English well, but I've seen Chinese people refer to that page as "proof" the phrase has made its way into mainstream English. If you see slang you don't recognise, Urban Dictionary can be useful, even for native speakers. Definitely don't rely on it though. If you're not sure about an entry, you should just Google it to see if anyone else has had the same doubts as you.

How much salt do you have? :) Although Urban Dictionary is an open collaboration, this also means that anyone can invent a word, call it slang and wait for other people to vote it up. In fact, most of the words are only "cool/funny things a friend of mine says" - no-one else really says that. There are a couple of useful insights, but it's best to treat what's written there critically, even sceptically.

It is actually hilarious... It surprised me by how funny it was XD