Mike,
"20 amp", "30 amp", and "50 amp" simply means how much current is available at a particular site, and each one will use a different style of plug. The 20 amp service will use a standard 3-prong household plug, 30 amp uses a large 3-prong plug, and 50 amp uses an even larger 4-prong plug. The amount of current available at individual sites depends on a few things: How large the wires are that run to the post, the size of the circuit breaker, and the type if receptacle installed (20 amp, 30 amp, 50 amp). Additionally, if the campground does not have 240 Volt service coming in to it, you can't get 50 amp service at all. Some sites may have one, two, or all three types of receptacles; you just use the one that your plug fits into.
As Tom said, adapters are available to allow you to hook up to any of the receptacles regardless of what your camper is wired for, but remember that if your camper has, let's say a 30 amp plug, merely plugging it into a 50 amp receptacle using an adapter will not give you 50 amp service in your camper. It's wired differently inside, and your main circuit breaker will still be rated at only 30 amps.