An explanation of outlet types for the various amp and voltage ratings:
The standard outlet in home wiring is the 15 amp outlet. It has two vertical slots and usually a ground hole. One slot is wider than the other. The wider slot, on the neutral side, is for plugs that have a wider stab and are called polarized plugs. The intention is to make sure that the neutral side of the appliance is plugged into the neutral side of the outlet. This insures among other things, that the appliance is dead all the way back to the switch to keep most of the appliance dead when the switch is off. The outlet can be installed with the ground hole up or down, although there are some safety reasons for going with the hole up configuration. (more on reversed polarity here.)
The 20 amp outlet would be installed where some appliances might be larger and require the higher amp rating. A 15 amp appliance can use this outlet. But the horizontal tab on the 20 amp plug insures it must be used in only a 20 amp outlet. The 20 amp outlet has a horizontal slot.
A reminder here, this outlet must be installed with #12 wire (not #14).
When you get up to 30 amp outlets, you will find that they are primarily 250 Volt outlets. Dryers use 30 amp outlets. There are 3 wire outlets, for 2 hots and a ground/neutral. And there are 4 wire outlets for 2 hots, the neutral and the ground. New dryers/homes will have the 4 wire, the older configuration is the 3 wire.
The 50 amp outlets will be be used in range/stoves. There are 3 wire and 4 wire applications here as well.