The defendant can post their own bail or ask a family member or friend to post it. If the defendant uses a bond company, the company may require the defendant to have a co-signer (someone who will help the company find the defendant should they fail to appear).
And while some will tell you that you can't remain anonymous, the short answer is actually yes, you can. (But with one major consideration.) The Court is going to need a name and person to return the bond to once the court date has been successfully completed.
Companies can issue bonds, but most bonds are issued by governments.
Defendants out on bond are often required to remain within a certain geographic area, usually within the state or county where the case is being prosecuted. Travel outside of this area is typically prohibited unless explicitly permitted by the court.
Essentially, it is when you break the rules of your bail, whatever they happen to be. Violating a bail bond is a severe offense, and courts will often take extreme action against you.