In most common law legal systems, such as the United States or the United Kingdom, a law enforcement officer must have probable cause to arrest someone suspected of committing a crime. The defendant, or person arrested, is then entitled to an arraignment or preliminary hearing before a judge or magistrate, at which time the judge will determine whether there is sufficient probable cause to hold the defendant and force him or her to stand trial for the crime. When a judge makes this determination, the defendant is said to be "bound over" for trial.
The exact requirements necessary for a defendant to be bound over for trial will vary from one jurisdiction to the other. In the United States, the prosecution must prepare a probable cause affidavit, or similar document, that summarizes the evidence against the defendant. The probable cause affidavit is presented to the judge in order to determine whether or not probable cause exists by which the defendant may be bound over for trial. The judge is not making a determination of guilt, only a determination that the prosecution appears to have enough evidence that the defendant should be required to stand trial for the crime.
Although not used as frequently as they once were, a grand jury may also make the decision whether a defendant should be bound over for trial. A grand jury actually listens to testimony from the prosecution regarding what evidence they are prepared to present at trial. If the members of a grand jury are convinced that sufficient evidence exists to charge the defendant with the crime, then they will issue a grand jury indictment and the defendant will be arrested and bound over for trial.
A defendant who has been bound over for trial may still be entitled to bail or release on his or her own recognizance. In most legal systems, a defendant is entitled to be released if he or she pays a bond as set by the court unless the crime is one of the few crimes for which a defendant is allowed to be held without bail, such as murder. If the crime is a less serious offense, such as a misdemeanor, the court may allow the defendant's release on his or her own recognizance, or by the defendant simply promising to return for all court hearings and not get into any more trouble while the case is pending. Regardless of the manner in which a defendant is released, once bound over for trial, he or she must return to court and face the pending charges.