With "Prosecutors" (also Prosecuting attorneys, State's Attorneys, District Attorneys, County attorneys, etc. depending on the jurisdiction, here referred simply as Prosecutors) we generally refer to a lawyer who, possessing a law degree and being admitted to the bar, represents the public prosecution, thus the State and the citizen, in a courtroom trial against someone who is suspected of breaking the law. Usually, Prosecutors are employed by a Prosecutor's Office, and can be either State-appointed or elected; they become involved in a trial after the Police has detected the suspect the evidence point to, and he or she is formally charged by the prosecution.
I say "generally" and "usually" because the Prosecutor's duties change, either slightly or drastically, from country to country, depending whether it has a common law system or a civil law system:
in Scotland, the prosecution is carried out by prosecutors on behalf of the Lord Advocate, and can direct the Police on the investigation. In Japan, prosecutors have large powers to investigate directly, directing and with the help of the Police.
In my own country, there were some prosecutors and judges who are considered heroes for their effort against Mafia but eventually got killed. Nowadays, prosecutors' work is being tampered and hidden by corrupt officers and politicians, their job denigrated: I've met lots of people who think that prosecutors just "throw a monkey wrench in the works". When people understand that it's not prosecutors, but THEY don't follow the law, then maybe we'll start to change.
Besides... let's face the other side of the coin too. I trust and hope most of the prosecutors here are good and faithful people who want to pursue the truth, but there are corrupt prosecutors who use their power to let criminals free in exchange for money. There aren't many things I hate as much as a corrupt prosecutor.