A prosecutor is an attorney who brings criminal charges against an individual on behalf of the state government and its citizens. They receive cases regarding people who have been accused of a crime, and review all of the evidence to build up a legal proceeding against them.
The prosecution of a suspect begins the moment that person officially falls under the investigation of any court. For example, if a warrant is issued to bring a person into custody, the prosecuting team will start to review the charges and evidence against them. Anytime a prosecutor feels there is not sufficient evidence to pursue a particular suspect, they may drop the charges and put a quick end to any criminal proceedings. As long as there appears to be a case against a suspected individual, the prosecution will continue to investigate, sort through evidence and deal with the police officers who are involved in the situation.
For lesser crimes, the prosecution may strike a deal with the defense team to settle the matter in early hearings before a trial begins. This is known as a plea bargain, and it will generally result in a fine, greatly reduced prison sentence or an alternative to imprisonment such as a time spent working at a community service program. The primary reason a plea bargain is made is to reduce the number of cases that are tried in court. Almost all court houses have a back log of pending matters, so dealing with minor cases in other ways are more efficient and effective.
When a criminal case does go to trial, the prosecution is faced with the duty of questioning witnesses, bringing forth evidence and attempting to convince a jury that the defendant is guilty. Their job is to prove the person is guilty of their charges beyond all reasonable doubt. Once the prosecution has finished their court room arguments, they officially rest their case and wait for the jury to make their decision.