I GOT YOU BRO!!
There are five primary positions to play:
Mid: Characters with good wave clear who can prevent the enemy team from pushing down the shortest lane. They are often easy to kill, and so are closest to their tower. Often mages and assassins go here.
Top: A character who is self sufficient and can go long periods of time without help from their team. Often tanks and juggernauts go here.
Jungle: They sustain off of killing monsters, and roam the map look for opportunities to save allies, catch enemies, and protect objectives. Often bruisers and fighters go here.
Carry (ADC): They are the most vulnerable and most powerful role. They go bottom lane with a support so they can farm gold and get strong. This role is almost always a marksman.
Support: They are a utility position that protects the carry early on, and later one supports the entire team in fights. They tend to do less damage, but are every useful. Often tanks and mages go here.
There are three primary things a champion brings to the game that define their class.
DAMAGE
DEFENSE
UTILITY
How much of these things, and how they do them defines their role.
DAMAGE CLASSES
-MAGE: They deal damage in bursts, often in large areas and at range. They are very powerful in team fights.
-ASSASSIN: They deal damage in bursts, often to single targets at melee. They are very powerful in 1v1 scenarios and focus on killing other damage dealers.
-MARKSMAN: They deal consistent damage over time, and will likely deal the most damage of any class in the late game. They stand back and deal lots of damage. They are the best at taking down towers.
-FIGHTER: They move a little deeper into the fray than most and look for opportunities to eliminate squishier opponents with decently sustained damage. Often they are good at taking towers.
DEFENSE CLASSES
-TANK: They have massive hp bars and resistances, making them hard to kill and difficulty to ignore. They hold the front line while the damage dealers do the killing.
-JUGGERNAUT: They are difficult to kill, while also dealing considerable damage. Often the centerpiece of team fights.
-BRUISER: Semi tanky, but also dangerous. They can slip through an enemy team and search out the squishies. They sometimes are hard to distinguish from juggernauts and assassins. It's a very poorly defined role.
UTILITY CLASSES
-UTILITY TANK: They protect the damage dealers with their bodies, and can often catch the enemy damage dealers so the team can kill them.
-UTILITY MAGE: They provide the team with ranged protection and crowd control. Some are dedicated healers.
There are plenty of other classes that are mish mashes of the above, such as mage bruisers and attack damage casters... but for the most part that should cover the important stuff.
