«How are you?» is a common way to greet people in the US and because it’s a greeting (a way to say hello to someone), people don’t really expect you to give a long answer. Just to give you an example, a supermarket cashier or a bank clerk might ask you «How are you?». The polite thing to do is to say something like «I`m fine, thanks, how are you?» and move on, even if you are not fine at all.
In the UK, some people use the word «All right?» to do the same thing — to say hello. When said quickly, it sounds like «aite?». Sometimes, the word «mate» (friend) is added — «All right, mate?» — «Yeah, fine. You?»
There are lots and lots of other phrases that can be used instead of «How are you?» — both as a way of saying hello and of actually asking how somebody is. Here are a few examples:
How are you doing?
How have you been? — usually used with friends, especially with friends you haven’t seen in a while
What are you up to?
How’s it going?
How are things?
What’s up?/Whassup?/Sup! — all versions of the same phrase and pretty informal
What’s shaking? — also pretty informal