Final boss is a term to denote the ultimate, or last, boss of the game. Bowser is one of the most famous final bosses along with Sephiroth and Ganon. In most games, after defeating the final boss, the game ends after a final cutscene (and sometimes a credits screen), however there are a few games that have a playable epilogue after defeating the final boss, such as Final Fantasy XIII.
The final boss will usually have more health than most bosses and sometimes changes its expression while getting weakened. Final bosses are also more likely than other bosses to have multiple forms. Ridely from the Metroid series has been a boss and a final boss in many video games from that series.