A boss fight is a concept that appears in many games, in which the player must fight against one or many enemies of generally high strength. A specific pattern of attacks is often needed, or 'weak points' may be found on the boss. Boss fights will usually increase in difficulty throughout the game, up until the final 'End Boss', who's defeat will often result in the end of the game.
Boss Fights have been around in gaming for generations and set a precedent for the way in which games were to flow. They brought in the concept of the linear gameplay leading up to specific boss fights at certain points. Originally besides just adding depth and length to the game and offering challenge, they were also often used for economic reasons. By giving a tougher fight and and almost taunt-like " game over" screen to coax the gamer to play again, this caused people playing arcade games to spend more quarters in an attempt to beat the boss or level, thereby giving the companies who had made the machines a bigger profit. While the trend has recently been to stray away from linearity, boss fights have remained a staple. However they now take on many different shapes and sizes as games have grown.
Most genres have customized boss fights to fit their specific requirements, for example a boss fight in a first person shooter is quite different than a boss fight in say a racing game. Commonly within games, the action genre in particular, a boss fight would often be pattern based whereby its weakness would be revealed in one of the recurring parts of the pattern whereby the Rule of Three would apply. As each genre offers a different type of control and different focus on gameplay, the boss fights have become a challenge in those areas. Such an example could be seen in the boss battles of Guitar Hero III: Legends of Rock, which has developed a mode specific to battling which is used to take on these "Legends of Rock". While boss fights tend to only be as good as their build up, it is the fight itself that can determine how much fun a boss fight is. They take a different approach than the typical gameplay type of fun, as the player is meant to be more concentrated and focused on smaller objectives. This is to lead to a sense of accomplishment for the player, which is perhaps why boss fights still exist today.