Players can purchase upgrades for their weapons using in-game currency, called credits. All purchased upgrades will last for the duration of the game.
If a player leaves during the game for any reason, providing they reconnect, their upgrades will be retained for 3 minutes. Also, their spot on the server will be reserved for that time period. Between waves, no robots will spawn whatsoever.
MvM's backstory is explained in the comics "Blood Brothers" and "A Fate Worse Than Chess".
In the former, Redmond and Blutarch Mann come to a truce, only to discover about their long-lost brother Gray, who kills them shortly after. In the latter, Saxton informs RED and BLU that they're out of a job, and re-hires them as a united force to combat Gray's robot army, beginning Mann Vs. Machine.
The boston maniac's job in MVM is to collect the money dropped by robots. Your speed will allow you to run around and pick up all the cash that your damage dealing team members can't get to. The best upgrades are resistances so you will be less susceptible to being killed when your bobbing and weaving through the robots trying not be targeted picking up loose change. Good weapons to support your team include the Sandman to stun bomb carrying robots, Mad Milk to help your team get health, and Bonk to distract big robots and give you an edge when picking up money. You don't want to be focused on damage output when playing Scout but don't shy away from shooting robots and picking off medics and snipers. It is also common to see Scouts use the Fan O' War in order to help deal critical hits to giant robots.
Good mvm maps for scouts: Mannhattan, Decoy, Coal Town, Rottenburg and Mannworks
The soldier has huge damage output and to help your team the most you should find a high spot with a blast jump and spam the robots below. Banners help a ton for you and your teams damage output against tanks and robots as well as the Conch helping them stay alive. huge opportunities for team play.
Good mvm maps for solider: all maps as solider is an all-around class
The pyro needs to be up close and personal with the robots. He can airblast the bomb holder back and reflect the dozens of projectiles shot by robots, as well as doing absurd damage to tanks. It is also common to see Pyros airblasting giant robots into pits of death, this is also good for resetting the bomb. The manmelter is a great option when faced with robo pyros as you can extinguish your teammates and get some CRITS.
Good mvm maps for pyro: any map with environment hazards (eg.Rottenburg, Coal Town)
Demoman you should lay stickybombs with the Scottish Resistance (increased sticky numbers) in combination with a Kritzkrieg medic at the beginning of the round. Demoman is also good at taking out enemy engineer's Sentry gun.
Good mvm maps for demoman: any maps with two main robot entrance (eg.Mannworks)
Heavies deal large amounts of damage and should focus on all robots excluding the tank, Heavies also should stay near ammo sources such as dispensers. Heavies should be at the frontlines with a medic. The Brass Beast shines in MvM with having a direct damage buff (even though being thought as a joke weapon in normal play).
Good mvm maps for heavy: any
Engineer is one of the best classes in MvM so make sure you build a sentry and dispenser, you rely on your team and the team relies on you. Rescue Ranger is a meta in MvM because of the healing from a distance. Building level-up canteen will help you rebuild your nest quickly, a good idea is to place all your buildings then use the building canteen.
Good mvm maps for engineer: any
Medic is incredibly useful in MvM being able to give kritz to teammates and protecting them with a shield! The Medic Meta is the Kritzkrieg and uber canteens (or stock with crits canteen). Medic should be healing his teammates 95% of the time with his power of overhealing his teammates.
Good mvm maps for medic: any
Sniper (not guaranteed as you might be vote kicked) should use the Hitman's Heatmaker because of the special ability being he doesn't have to unscope with a full charge. A sniper should get explosive headshot first than get reload speed and fire rate.
Good mvm maps for sniper: a small spawn for robots(eg. Mannhattan, Bigrock, Decoy)
Spy, your main objective is to sap ubercharge medics or waste ubercharge, so Ambassador and Cloak and Dagger will protect you from death or anything else. Using ammo refill canteen can instantly refill cloak bar and Sapper.
Good mvm maps for spy: Bigrock, Mannhattan.