They are also a little quieter. They're easier on your fingers when you're first starting out. Electric steel strings are designed to sound good through a pickup, so they're different from acoustic steel strings. The first three strings are plain steel, and the other three are usually nickelwound. They don't sound very good on an acoustic guitar, and acoustic guitar strings don't sound very good on an electric, although there's no reason you can't use them that way if you want to.
Nylon strings are the modern version of gut strings, used for classical and flamenco guitars.
Usually the first three strings are plain nylon, and the rest are nylon wound with silver wire. They have a much softer tone than steel strings. A steel guitar string's diameter is called its gauge . Sets of strings are graduated so that their tension is as consistent as possible when they are in tune, to offer an even resistance when you play them. Every manufacturer has a different definition of what those labels mean, so don't expect Dean Markley extra light strings to be exactly the same gauges as Gibson extra light strings.