I believe that bankroll management (BRM) is what separates the good players from the bad long term in poker. Then why is BRM so important? Well for starters, when you got no money, you can’t play.
So what is good BRM? The important thing is to play where you can afford to lose without jepordyzing your BR. It should not be possible for you in any given session to lose more than 10% of your BR in a cash game (general rule). If you lost the last few sessions you played you might have to go down in limits and recover before stepping up and continuing building your BR at higher stakes. A good player realizes this and steps down, a bad player does the opposite and steps up to win his/her money back. It might work that one night, but not in the long run.
It is also important to analyze your style of play before determining how much BR you need for a certain limit. Maybe you are a very lose and “crazy” player that like to “gamble”, or maybe you like to play heads-up or short handed games. These factors increase your swings and that should of course mean that you need a larger BR for playing at a certain limit. If you are tight, play 10-handed games and don’t bluff much, then maybe you can do well with a smaller BR. Maybe you heard general rules like “you should have at least 20 buy ins at any given limit” but I argue that it is more important to analyze your style of play to determine how many buy ins you need. It is also important how many tables you play. The more you play the larger BR you need.
If you are playing touraments, you should not buy in for more than 1/50th of your BR if you want to give yourself a chance to win money before going broke.
Sometime maybe it is even more important not to go broke in poker than to actually make money. Cause the players that never go broke can keep playing poker for a long time, and that is what we all want to do, right? 
-amexos
