Traditionally, two teams of two attempt to toss a pingpong ball in every one of their opponents' cups (usually six, arranged in a triangle) before they do the same, in turns. But there are many, many variations here.
Pros: I mean, it may be the most classic of all the drinking games (aside from "chug the beer"). It has a fairly small learning curve, requires almost no movement, and is also a great way to consume multiple drinks in a short period of time, which -- depending on the tailgate -- might be exactly what you are looking for.
Cons: Well, you need a pretty big table (or, an actual tailgate) -- because the whole "let's stand really far back on a small table" method just isn't the same game. You also need pingpong balls.... and those pingpong balls tend to get lost under cars when you play in a parking lot scenario. Also, beer pong tends to bring out a certain, fratastic style of douche-baggery out in even the most mild-mannered individuals. Play with caution, folks.