When a person is arrested, he is brought before a judge for what is called a first appearance hearing. At this hearing, the judge makes an initial determination of bond, the conditions of the person's release from jail. This could include a monetary bond to be posted, and other conditions, such as a restriction on travel or an order not to have contact with the crime victim, etc. In most cases, a person who is arrested is entitled to have a bond set. There are very specific rules for when a person can be held "no bond," meaning that they cannot get out of jail under any conditions pending their trial.
Once that initial bond determination is made at the First Appearance, it's still not set in stone. A person can appeal to the trial judge to reduce his/her bond or grant bond if none was origianlly set. The factors that a judge will use in determining whether or not to reduce or grant a bond are things like the danger the defendant poses to the community, the defendant's financial situation, and the risk of flight.
In the case of George Zimmerman, the Florida man accused of shooting and killing Trayvon Martin, an unarmed 17-year-old, his bond was set by the trial court after a bond hearing at $150,000. But that was before the prosecution and the judge knew about the $150,000 that Zimmerman raised in public donations for his defense. Neither Zimmerman nor his wife disclosed to the court at his initial bond hearing that they had $150,000 stored away in a bank account. Once this was discovered, it certainly amounted to a change in circumstances that warranted a revocation of his bond, which was done. Today, the issue of bond came up once again. Zimmerman once again went before the judge to determine if a bond would be set in his case. This time, the judge set bond at $1 million, certainly substantially higher than was previously set.
Time will tell if Zimmerman is able to raise the capital needed to bond himself out or if he remains in custody pending his trial. Either way, it will surely be an interesting trial for those observing it from the outside.