Tiny stack - 12 big blinds or less
When you have a tiny stack and decide to bet or raise it should always be all-in.
The reason for that is that you will have committed so much money to the pot so you would in most cases have to go all-in on the flop anyway.
Going all-in before the flop forces your opponents to make difficult decisions.
Small stack - 25 big blinds or less
Your goal is to be all in either before the flop or on the flop when you play.
But this stack size is too big to make moving all in a good move unless someone has already opened the betting.
If someone has raised before the flop and you have a hand good enough to reraise all in that is generally a good play.
If you get to the flop and have catched some part of it you can generally aim to be all in.
Medium stack - 100 big blinds or less
With this stack size you will have more difficult decisions playing on the turn and the river. Most of the time you will not be all in.
It is not possible to make a simple strategy that will help out in all situations that you will face with stacks of this size.
This strategy has been developed to help you along the way and make some of the difficult decisions a lot easier.
Deeper stacks - more than 100 big blinds
This is not for beginners. The decision making becomes very complex with a large stack size.
Sometimes you will find yourself at this stack size when you play tournaments whether you like it or not.
Then you should play very tight (play few hands) and be careful. But if you find yourself holding a really good hand, don't hesitate to put pressure on your opponents.
Soon you will find that the blinds have raised and that you are at a more comfortable stack size.