Whats the difference between all the trackmania games? I don't get it

Trackmania 2 player here. At first sight they feel/look pretty much the same. But when you start playing them you clearly see the difference. But I'd suggest you pick-up Trackmania 2: Canyon as it's in my eyes the best one. But if you really liked the older Trackmania games, get Trackmania 2: Stadium.

The reviews from people who claim they are the same game are the same people who would say Assetto Corsa is the same as for instance Forza 5. And yes, every game has it's different servers.

Sorry for hijacking, but is there a reason I should favor one of the newer TrackManias (apparently Canyon is the best) over the free TrackMania Nations Forever? I only play casually and even though it's cheap, I don't see an incentive to ... upgrade?

Stadium is the same (almost) as that free Nations Forever game, others are different. Canyon is awesome game, haven't got Valley yet :(
Don't know why i bought Stadium :D

Trackmania Nations Forever is a free-to-play version of Trackmania United Forever. The former can only play in stadium mode (which features a super-grippy open wheel car designed for crazy stunts) and forces you to sit out every fourth online match. If you buy United Forever (the premium version), not only do you not sit out every fourth match, you get six new race types: Desert (a canyon-type series where you drive a car with intentionally bad suspension in an attempt to move quickly without tipping over), Island (really fast driving in an oceanic environment, relying heavily on drifting to keep up speed), Rally (a jungle-type series that involves a vehicle designed for offroad), Bay (a metropolitan environment driven with an SUV), Snow (a slick environment driven with a 4x4) and Coast (another island type environemnt that relies more on winding roads with a car that is intermediate in ability)

Trackmania2 Stadium is a remake of the Stadium in Trackmania with more features and better graphics. Trackmania2 Canyon is a succcessor to the gameplay of the Island enivornment, set in a desert valley. Trackmania2 Valley hybridizes the gameplay of the Rally and Coast modes in a suburban environment.

If you're just getting into Trackmania, I'd suggest testing out Nations Forever, then buy United Forever. T2 is nice but the variety of modes in United Forever is fantastic.