Games played against other human beings, however, must be fair. Even when teams are not exactly symmetrical, such as in Left 4 Dead's survivors vs infected, every effort is paid to balancing the experience for players on both sides of the game. The best players must win by virtue of skill alone.
Of course, that's not always the case, because when cheats appear, it's not by design, but by malign intention.
In online gaming there are two major modes of cheating. The first of these are loosely termed 'hacks', because they're often a manipulation of the game code, or some kind of software tweak that gives the player who uses them an unfair advantage.
The most widely known in online shooters is the wallhack, which allows players to see where their enemies are throughout a level (interestingly, rather like the infected's ability to see each other and players through walls in Left 4 Dead), as if the walls were translucent.
Such hacks are relatively easy to develop, and can be easily downloaded and installed by players. Sometimes they're modifications to the game itself, other times they can be additional programs that alter how graphics are rendered.
Other hacks can be more sophisticated still, giving players powers and abilities in the game that they should not otherwise have – something that comes about when the cheaters, or the people who implement cheats, get their hands on the client code of the game installed on their computer.
And you can't always tell who is cheating, either, because such exploits may simply allow the player to seem like he is playing really well, when in truth he has an unfair advantage of some kind.
Bots are a similarly hi-tech unfair advantage. In shooters these are programs called 'aimbots' which detect opponents are then fire very accurately. They're really rather sophisticated, and can be used to devastating effect by cheaters.
Just one player using an aimbot can massive alter the outcome of a game, especially where pin-point accuracy is required, such as when using sniper rifles and railguns.
Bots are also used in MMOs, where the player sets up a bot to perform game tasks such as killing spawns or running simple quests. It's possible to set these up to 'farm' certain resources, which can be extremely infuriating for gamers trying to play the game proper.
Many MMO players will have encountered them, often with characters killing the same targets over and over. Most famously, one Ultima Online player claimed to have purchased a house by trading the gold he earned from running a dozen bots on six PCs over a couple of years.