A petit jury is a trial for civil and criminal cases. The petit jury listens to evidence presented by both parties during a trial and returns a verdict.
A grand jury does not determine guilt or innocence, but whether there is probable cause to believe that a crime was committed. The evidence is normally presented only by an attorney for the government. The grand jury must determine from this evidence whether a person should have formal charges filed by the government. If the grand jury finds probable cause, then it will return a written statement of the charge called an indictment.