A not quite ubiquitous but very commonly used phrase - most often in the Generation X age group (and younger) - alternate spelling "Whoo Hoo!". The literal translation approximates "I am quite pleased with my circumstances". The origin of this phrase has often been said to be Homer Simpson - but this is not entirely correct. Homer actually took a seldom used phrase and propelled it into widespread popularity among The Simpsons' viewing audience. It is usually pronounced almost as a single syllable. The tonality is almont always falsetto, and remains monotone until the last part of the phrase where the tone drops down quickly.
Beyond the apparent simplicity of the phrase, there are more subtle meanings and variations that distinguish it from other common near-antonyms such as "Yes!", "Alright!", and "Yeah Baby!". Woo Hoo is almost always used to express personal gain in some way rather than simple enjoyment of an observed event or circumstance such as a good football game, etc. Some examples may help to clarify the more subtle uses.
As you can see - the most common usage is a spontaneous public expression of delight in your good fortune, in a successful outcome, in a difficult accomplishment, or in anticipation of an impending reward. The phrase is meant to share your personal joy with those around you, or to announce your pleasure in front of opponents in a non-gloating way (Additional actions may change this to actual gloating.) It is useful to examine circumstances as well where a loud "Woo Hoo!" may not be appropriate.
- You are trying to catch the same taxi as another guy - while running, he trips and breaks an ankle.
- You are playing an intense game of Risk and one of your opponents wipes out the other.
- Your roomate asks out a cute coed and she says yes.
- You are watching an action movie in a crowded theater - the bad guy in the film gets shot by the good guy.
None of these circumstances really has anything to do with something you have done, merely the observed misfortune or fortune of others. A real "Woo Hoo!" should be have personal meaning and intent to share happiness with those about you.
There is another side of "Woo Hoo!" which is darker and more subtle but quite useful as well - the sarcastic "Woo Hoo!" Said with a sardonic tone (and a period rather than an exclamation point), this simple phrase can communicate volumes to those you address. Essentially, you are saying that you are underwhelmed either with their response to you or with your current circumstances. Perhaps the most appropriate example of this use would be the following story:
Two friends, Nate and Tom were walking the streets of Washington DC late one night after an evening at an expensive restaurant. Nate had paid about two-hundred dollars for the meal by credit card with the Tom promising to pay him back in cash later. They decided to take a shortcut through a back street to their destination, and were accosted by a rough-looking but reasonably calm man who held them at gunpoint and quietly demanded their cash. As they pulled the money from their wallets, Tom paused, handed his cash to Nate, and said "Here, Nate. There's the hundred bucks I owe you."
ahem - woo hoo.