Simply put, a bail bond is a monetary guarantee for the Court that a defendant will appear at all times they are ordered to while their case is pending. When a person goes to court as a defendant in a criminal case, bail is set by the judge. If they are arrested and have not been brought to court, bail is set according to a schedule. The bail can be posted in full in cash, or by posting a bail bond through a bail agent. The bail agent collects a fee (usually 10% of the amount of bail) and some sort of collateral for the full amount of the bail.
Collateral is usually a piece of real estate, a car, a bank account, or something else of value that the bail agent can hold onto while the defendant goes to court on the case. Depending on circumstances, the signature of a qualifying cosigner can be used in lieu of collateral. When the case is completed, as long as the defendant has not failed to appear, the collateral is returned to whomever deposited it with the bail agent.
What happens if a defendant fails to appear.