What I call an investigative judge is in charge of the enquiry. He gathers evidences about the case, both against the suspect and exculpatory, on his own or sometimes at the request of the prosecutor or of the suspect’s lawyer, give orders to the police, leads searches, hear witnesses, etc… If he believes that there’s a case against someone, he sends the case for review to a court that decides whether or not the accused should be tried (what an american grand jury does, if I’m not mistaken).
His job stops there, and he doesn’t take part in the trial (in rare instances, he might be called as a witness, which generally means that he appears to have seriously botched his job, and that the court wants him to do some splaining about it).
Basically, he’s doing what the prosecutor does in a common law system, except that the common law prosecutor search for evidences proving his case, while the civil law investigative judge is required to search for all relevant evidences, including exculpatory evidences.
What I call a trial judge is…well, a judge present at the trial. Using the evidences, written testimonies, etc… gathered by the investigative judge, he presides and conducts the trial, hearing witnesses directly and overseeing their examinations by the lawyers/prosecutor, presenting the evidences, ordering new investigations/expertise if they appear necessary during the trial, directing the jurors if there are jurors, etc…
As mentionned above by another poster, his job (or more exactly the job of the court he is part of) is to actively seek the truth during the trial, not just to be a kind of referee between the prosecution and the defense, like in the common law adversarial system.
An investigative judge can become a trial judge during his career (or the other way around), at least here in France. But at a given point in time, a judge is either one or another (only conducting enquiries or only presiding trials).
There’s no investigative judge in Italy. According to wikipedia, they’ve now become “judges for the preliminary investigations”, with apparently a role limited to make sure that the prosecutor in charge of the enquiry doesn’t violate the defendant’s rights.
Wikpedia seems to have a rather complete entry about the Italian criminal procedure , more relevant to this thread that my descrption, since, as I already said, the Italian procedure has evolved considerably towards a much more adversarial system.