As is a word that compares two things. Long describes length. It's a phrase that compares two lengths (physical dimension or time duration), but it can also define a conditional statement.
This banana is 18cm long. My hand is 18cm long. My hand is "as long as" the banana. (Physical dimension)
In Matt's example of staying in america, it is also length, but of time duration. "I will study english for the same length of time that I stay in america. 'As soon as' I leave america, I will stop studying english." Meaning they will continue both for the same duration and stop both at the same time.
As soon as - an event that happens at the same time as what is compared.
As long as - the same length (physical dimension, or time duration) as what is compared.
It can be used as a conditional as well. "As long as you work here, I will pay you." It is describing length of duration, but it is also describing one action (me paying you) is conditioned upon the first statement (you working here). If the length of time of your working here comes to an end, so will the length of time that I pay you.
Nhan's example of "as long as you're here" is yet another use. This has nothing to do with the length of duration. It indicates "because you happen to be available to me". You could replace "as long as" with the words while or since. "While you are here, could you help?"