What exactly is an FBO and what services are provided?
A Fixed Base Operator (FBO) is an entity - usually a commercial company - which provides "aviation services" at an airport. One of the main services they provide is fueling, either full service or a self service pump.  They don't all provide both 100LL and JetA, but most do.  

Larger ones provide other services, such as catering, lavatory service, pre-flight briefing computers and wifi access, ground power units, and tie down or hangar space for parking.  Charges vary from place to place, most large FBOs will charge a fee for aircraft parking, but sometimes waive a day or so of that if you purchase fuel.  They can often assist in arranging ground transport or hotels as well.

Most are open hours like 7AM - 8PM, but that varies as well.  Most self-service pumps can be used 24 hours a day.

Some have a "crew car" available to borrow for a short trip to get lunch or run an errand, though calling ahead to check on that is important.