American Airlines operates flights out of two terminals in LaGuardia. Terminal B or the C Terminal which used to be where US Airways flights departed. Most of the Terminal C departures are legacy US Airways routes as well as some Express flights. As I was flying to RDU and then connecting on to London, I was slightly surprised to be flying out of this terminal.
Check-in procedures can be done in the Central Terminal or Terminal C, but security must be cleared at Terminal C.
After TSA, it's a short walk to the lounge, that while small and efficient has soups/snacks and a full bar (premium beverages/wines are available for a fee) that serves well for a pit-stop before your flight.
Lots of seating areas, that can get a bit crowded during the early hours and late afternoon.