The Machines are the enemies fought in Team Fortress 2 Mann vs. Machine, during missions. They were created by Gray Mann in order to destroy everything created by Mann Co.
Regular robots function nearly identically to the players, being operated by bots, whilst special robots exist that use non-Stock weapons, and other forms of robots exist with abilities and directives that no class normally possesses. Unlike players, robots can notably be sapped, temporarily disabling them often in groups, or in the case of giant robots, slowing them. As noted by Miss Pauling, they run on money, and they drop it when destroyed, allowing players to use that money to upgrade their weapons or buy a Power Up Canteen. Robots follow a set path to the bomb hole, marked by glowing blue arrows before each wave, though they may stray off that path if the bomb carrier is pushed away or to pursue a player. Robots will occasionally taunt after killing a player, and the bomb carrier will always taunt to receive its buff. Robots have an infinite supply of ammo and do not drop their weapons as pickups on death. Robots however also never randomly Crit, although some have a means of gaining the Critboost effect.
One robot per wave will enter the field armed with a bomb. The primary directive of the robots is to transport this bomb to the hatch at the end of the map, and the Defenders must stop them. After the bomb is dropped, another robot can pick it up. Any robot can carry the bomb, excluding the Sentry Buster and the Tank (although the Tank carries a separate bomb). Spies and Snipers won't actively try to pick up the bomb, but will if forced on to it via knockback. Carrying a bomb will slow a robot to about half speed, except Giant Robots. After all robots in a wave are killed, the wave is completed, while if the bomb is transported to the hatch and detonated, the wave is lost.
Robots have rudimentary behaviour. Their main objective is to either pick up the bomb and follow the preset path to the bomb hatch, or escort whichever robot is already doing so. On the whole, they do not take alternate routes to the bomb unless the sub-wave is programmed to flank, or the bomb has been dropped in a location off the main path. Robots generally do not care about destroying Dispensers and Teleporters if they are not in the way. They are also completely oblivious to cloaked and disguised Spies: they ignore flickering cloaks, collisions with "teammates", uncloaking noises, and possible Dead Ringers. They can detect if a nearby teammate has been backstabbed, but not with regularity. They do however always notice when a Spy is on fire.
Robots receive certain buffs after a period of time carrying the bomb, with new buffs stacking with the previous ones. A graphic on the HUD indicates which stage the bomb carrier is on and how long until it receives the next buff. Upon receiving a buff, the robot carrying the bomb will taunt with an accompanying sound cue, and surrounding robots will halt to wait for the carrier. The defending team can and should use this as a window of opportunity to kill the bomb carrier, especially if the robot is getting the third upgrade. These buffs will not occur if the bomb carrier is a Giant, though the HUD graphic shows a full meter.