In many judiciary systems (including that of the United States), a prosecutor is a member of the justice system who is responsible for presenting the case in a criminal trial against an individual accused of breaking the law.
In principle, the job of a prosecutor is to simply present the facts, the things that the accused is being accused for, as well as the evidence and the reasons why he or she is suspected for the crime. Also, most importantly, in principle the job of a prosecutor is to make sure that justice prevails, and that people get a fair trial, and that the judge and the jury (if there is one) get all the facts in an unbiased and clear manner. Unlike some people might think, a prosecutor's job is not to try to convict the accused; it is to simply present the case against him in the most factual and unbiased manner, so that the judge and/or jury can get all these facts in order to make a fully informed decision. (The reason why it might look like a prosecutor's job is to try to convict the accused is because his task is to present the facts that the defense attorney obviously won't present. His job is, basically, to present the other side of the story.)
This is the theory. The practice is a bit different, especially in many parts of the United States. There, many prosecutors seem to "collect" convictions, as if they were badges of honor or something. They seem to have this notion that their proficiency as prosecutors is measured by how many convictions they get, and thus they try to convict as many people as possible. It doesn't even matter if those people are actually guilty or not. Being considered a "tough prosecutor" in terms of how many people they have successfully convicted seems to be something to aim for.
Not only have they completely distorted their own role in the judiciary system, they go much further than that. In fact, they often resort to underhanded tactics in order to get convictions. They do not care for justice, or whether the accused really is guilty or innocent, all they care about is if they get a conviction, any kind of conviction.
One of the tactics they use is to scare the accused by overcharging them. This means that they are charged with a ridiculous amount of crimes that don't even make sense (such as, for example, charging them for "making terrorist threats" if they, for example, were involved in a bar fight.) Thus they offer a deal with the accused: Just plead guilty for a minor crime, worth of something like 2 years in jail, or risk trial and being convicted for 30 years or the like. Good defense attorneys would see through this underhanded tactic and never let it fly, but most people, especially the poor, cannot afford good attorneys, so the attorney may be inexperienced, incompetent, or too scared to go against the notorious "tough prosecutor" (or, to be frank, just outright lazy, because it's much easier for him or her to just get a quick deal than to go through a lengthy trial that requires tons and tons of work.)
This is a travesty and a mockery of the judicial system. The prosecutor's job is not to convict people at all costs. It's to see that justice is served fairly. A prosecutor should be proud of his or her job when the guilty are convicted but especially when the innocent are acquitted. "Innocent until proven guilty" should be the highest motto of a good prosecutor.