I am not a criminal lawyer, I just play one with minimal adult supervision.
What
skylark said is pretty much correct.
I've clerked for two different prosecutor's offices (one juvenile division in a big city, one everything in a county seat) and generally what happens is something like this.
1) Virginia Victim calls up the cops and says "Steve Suspect just assaulted me!"
2) Cops come over, take a rape kit of Victim, interview her.
3) Cops write a big long report detailing everything they did, then generally say "we recommend charging Suspect with 2nd degree sexual assault."
4) Prosecutor reviews the police reports, decides how to charge the Suspect based on the evidence, and writes and files a formal complaint listing the charges against him.
5) Prosecutor and Defense Attorney fight about this for a while then either settle or have a trial.
In a case like this, he probably would have been tried for rape and sexual assault and the jury only found him guilty of sexual assault (since you said he was tried.) Otherwise, sometimes they'll cut a plea where if he please to sexual assault the prosecutor will drop the rape charges.