What is a Poker Bankroll?

When poker players talk about their bankroll, they mean the amount of money that they have - one way or the other - set aside as their poker capital; the amount of money that they are able to stake at the table at any time. There are different views on this, and although it's dangerous to generalize, I feel it is justified to say that any serious poker player will have a pool of money that he or she has dedicated to playing poker with. This is the bankroll.
How you handle your bankroll and where you keep it is not all that important. It can be a tin-can of quarters under your bed, or money invested in stocks, or more commonly a separate account at your bank. The important thing is that you can access it and that you know how much is there - and for the sake of score-keeping, a separate bank account is clearly preferable.
There are many reasons to play poker, not the least of which is to have fun. If your ambition at the table is to enjoy some excitement, or to socialize with other players, then a bankroll isn't the most important part of your game. In fact, you may find it a boring chore or maybe the mere thought of having to deal with keeping track of how much you've won or lost seems intimidating. Maybe you're rich and the limits you play at are simply so low in comparison to your wealth that even a maximum loss will not be noticeable to your spending money. If any of these descriptions apply to you, then it's possible that any advice regarding your bankroll will be of no interest, but I want to try anyway - and I encourage you to read it regardless - because I believe that you will get more enjoyment from poker if you carefully manage the parts around it. The reason I believe this is because most goals that you set (that have measurable success, as good goals should have) are tied to your bankroll, and being able to measure success - explicitly put a number on it - is a good way of being sure that you've achieved what you said you would. Achieving things is enjoyable!
And for those of you who want to play serious poker, starting a bankroll is just about the only way of getting there. Or are you going to make a $600 buy-in at a limit table using your grocery money? You need to separate the money you use for day-to-day things and the money you play poker with. With some work and skill, you will notice something interesting happening after a while: The reason to keep your bankroll separated from your checking account transforms from being to protect your checking account from poker losses, to protecting your bankroll from impulse spending!
So regardless of why you play poker, I think you'll have a good reason to keep your poker money separate from your every day money. Start treating your poker money as a proper bankroll - you won't be sorry you did.