The easiest way to describe a sidestroke is by using pictures, so here's a couple:
Basically your body forms a T-junction angle with the water - you spread your arms at the same time you spread your legs. For towing purposes (or to save someone's life) you actually use the arm that is over your legs and stroke with the forward arm. Since your legs are scissoring sideways, they won't hit the person you're saving, and your body will hold the other person's face out of the water as well. (It's what Diana used to save Steve in the swimming scene in Wonder Woman, if you were paying attention. )
This is usually the last stroke they teach you in swim class, because the first priority of most swim programs is you being able to save yourself from watery drowning doom first - for that, freestyle is the best choice, unless you are in a river or extremely exhausted.
Backstroke is moving your arms in freestyle motions - it's freestyle on your back. Elementary Backstroke is breastroke on your back.
Elementary Backstroke: (also known informally as the Chicken-Star-Rocket)
Backstroke is the second-fastest stroke after freestyle and is commonly used for flip-turn practice because of lack of the need to regulate your breathing going into and out of the turn, provided that you don't hit your head while doing it.
Elementary Backstroke is a stroke that's commonly used for relaxing after a long time swimming. It's an extreme survival stroke used to conserve energy. If you are in the middle of the ocean or you got thrown into a huge lake from a helicopter after a hard fight against a bunch of evil bad guys, this is how you get to the shore alive - one easy relaxing glide at a time.