IMDb registered users can cast a vote (from 1 to 10) on every released title in the database. Individual votes are then aggregated and summarized as a single IMDb rating, visible on the title's main page. By "released title" we mean that the movie (or TV show) must have been shown publicly at least once (including festival screening).
Users can update their votes as often as they'd like, but any new vote on the same title will overwrite the previous one, so it is one vote per title per user.