A bondsman is the person you call when you get arrested, and don't have enough money to get yourself out of jail. They are not exactly a get out of jail free card, but they are better than rotting in jail while awaiting your trial.
A bondsman will have filed paperwork with the local government allowing them to sign over responsibility for a persons bail money. They don't do this for free; they will expect a fee for their services. The fee is usually ten percent of the total bail amount. So if your bail is $10,000 (as mine was the only time I got arrested), then a bondsman will expect $1000 for their services. You can usually attempt to negotiate this fee, although most bondsmen will not drop the total price. However, you can usually find a bondsman willing to take payments, as long as you can give them some money up front. They are willing to do this because signing for your bond is like free money to them (as long as you don't run away).
Never ever screw over a bondsman. The nature of their job brings them in contact with criminal types on a daily basis. It is not unheard of for a bondsman to send over a few thugs to help "collect" a debt. As long as you pay your money on time you will be fine.
Becoming a bondsman requires a large amount of paperwork (and some pretty good finances to boot). Call your local Government Center if you are interested in this line of work, they should be able to direct you to the correct office to begin the process.