It's basically a show of five worthless, clueless assholes who own their own bar, but somehow it's hilarious nonetheless. Danny Devito joined the cast after the first season or two, bringing a familiar face. Otherwise, everyone else is pretty much unknown (though you may have seen Mac on season 3 and 6 of Lost). It started as a pilot written by two of the actors and shot for less than $200. FX picked it up as a series based on this cheap pilot from unknown actors/writers, and they're now into season 6.
Wikipedia got it right:
The series follows The Gang, a group of five depraved underachievers who run Paddy's Pub, a run-down bar in South Philadelphia. They are dishonest, egotistical, greedy, unethical, lazy, arrogant, and antagonistic, and are often embroiled in controversial issues. Episodes usually find them hatching elaborate schemes, conspiring against one another or others for personal gain or the pleasure of watching their downfall. Their tactics often rely on inflicting emotional and sometimes physical pain on individuals both deserving and undeserving. They regularly use blackmail and manipulate one another and others outside of the group. Their unity is never solid; any of them would quickly dump the others for quick profit or personal gain regardless of the consequences. Almost everything they do results in competition between them.
