Just a couple of things to add to CapnZapp's thorough summary of the stealth rules.
1. While one is considered in cover against ranged attacks when one's allies are between you and the "shooter", said cover does not count so to allow you to to remain hidden. AFAIK, this is the only instance where cover does not count.
2. Since combat adantage from being hidden lasts until the end of the action which cancels ones "hidden" status, one can get combat advantage on attacks if:
- they are done from range (be it through a ranged, burst or blast attack - or if you are using a weapon with sufficient reach)
- if somehow you are adjascent to the target (it may have moved up to you, provoking your readied action, or you may have snuck up to it through cover)
- by charging the target, since a charge (even if using a double move) is only a standard action. This will typically restrict you to only making a basic attack, however.
- by some power (CapnZapp cited a couple) that allows you to move up to a target and hit it as a single action.
CapnZapp caught many of these, but I felt he missed what I think would be the most common method - charging.