@TreeMan - I believe most cases are decided by a grand jury. I don't know if different states have different standards, but I know that in certain circumstances the prosecutor can charge someone without grand jury approval.
Probably the most prominent recent case where this happened was with the Trayvon Martin shooting. In Florida, the only cases that are absolutely required to go in front of a grand jury are first degree murder charges. Since it did not qualify for that, the attorney had to option to go to a grand jury or make the decision herself.
A lot of people criticized the move saying that George Zimmerman should have been arrested immediately. Regardless of whether or not the prosecutor thought he was guilty, she had to make sure that there was adequate evidence to make the charge. Otherwise you end up with a prosecutorial discretion problem like outlined in this article.