Generally, Pennsylvania Rule of Evidence 404(b) prohibits the introduction of a defendant's prior bad acts including criminal convictions. This means that the government may not introduce such evidence in order to convict someone of a crime. There are several reasons behind this prohibition; including to prevent a confusion of the issues and to ensure a defendant is not convicted of the current crime based on past behaviour to which there may be no connection.
There are certain exceptions to this rule, however. Crimes of crimen falsi are sometimes permitted, for example. A crime of crimen falsi is a crime of lying such as fraud or theft. Such crimes go to the credibility of the defendant to tell the truth. Also, if the prior crime has some relationship to the current crime it may be admitted.
It is important for any criminal defendant to be aware of his/her prior record and to discuss it with the attorney prior to trial to ensure there are no surprises.