Spot the difference between these two outlets:
The one on the right has a T-shaped slot in place of one of the two blades. As everything in engineering, this is not random. This extended slot gives you (or should be giving you) useful information about your electrical connection. It turns out that an outlet with a T-shaped slot is configured to feed up to 20 amps of current while the regular two-blade outlet can guarantee at most 15 amps (of course, if the outlet is not installed to code, this may not hold).
What I like about this, though, is that the 20-amp outlet is backwards compatible for devices that don’t need as much power, i.e. you can plug in a device that only needs at most 15 amps into a 20-amp outlet (which is fine, an outlet will give you as much current as you need, up to a limit). However, you can’t do the reverse, i.e. plug a device that needs 20 amps (presumably if I equip a device with a plug with a T-shaped prong, I’m claiming that 15 amps is not enough) into a 15-amp outlet — its plug will simply not fit into a two-blade outlet! This is a great safety feature.
Unfortunately, it doesn’t seem like the more advanced outlets follow the same convention.