The side kick is one of the most effective kicks in the martial arts and for self defense. There are quite a few methods in how it is utilized, such as, offensive, stationary, and defensive.
Offensive Side Kick – The purpose of the offensive side kick is to generate enough power and momentum to take out your opponent.  It can deliver significant damage if executed correctly, and can even have the potential of injuring someone permanently.
The offensive side kick is a technique that should be executed with caution.    You do not want to throw this kick from across the room.  It is highly telegraphic, and if you miss the attacker, you will be set up for a counter attack.  However, if executed correctly and appropriately, can neutralize a threat rather significantly.
Defensive Side Kick – The defensive side kick is a great tool when you are trying to gain distance from your attacker.  There is no significant power like the offensive, but is a great kick to use with follow up techniques such as another side kick or a punching combination.
Stationary Side Kick – This is how the side kick is started;  by a stationary position.  The stationary side kick is the building block where all other side kicks are developed.  It also helps to learn how to execute a side stomp to disable an attacker.
These are just three ways that a side kick can be employed.  But the side kick can also be used to create more advanced methods, such as, the turn side kick or jump reverse side kick.  The side kick is my signature technique, and have used it in plenty of tournaments as well as street self defense.  No matter what types of techniques you learn in your training, make sure the side kick is one of them.