Early Action - You apply before November 1st to receive a decision by December 15th. You can either be rejected, deferred, or accepted. You can choose to accept their offer, or apply to more universities and decide later on where you want to go. But the general rule is that if you apply Early Action, the school should be your first choice. Yale and Stanford offer Early Action. Other schools include UChicago and MIT.
Early Decision - You apply before November 1st to receive a decision by December 15th. You can either be rejected, deferred, or accepted. However, if you are accepted, you MUST accept their offer and attend that school the following fall. You should only apply Early Decision if you are 100% sure that this is the school for you. Most schools offer ED, but I know that Harvard and Princeton got rid of it.
Single Choice - this usually applies to EA, where you can only choice one school to do Early Action to. Stanford and Yale are Single Choice Early Action schools.
Most people think that applying early will give them an advantage in the admissions process, and although the acceptance rates are usually higher, you have to take into consideration that many of these applicants are also of higher caliber. Also a lot of them may be recruited athletes or legacies.
Generally Early Action doesnt give one much of an advantage because so many people opt for EA since it's non binding. If you're looking for an advantage, I'd say Early Decision would be a better choice.
Hope this helps!