Walls/tanks/sweepers

Sweeper :

A sweeper or more commonly known as "Glass Cannon" is a role, that benefitts from high speed and high atk/sp.atk so there are 2 versions of Sweepers.

1.Physical Sweeper :
EVs : It would be best to go for 252 atk and 252speed and 6HP.

Abilities : Accuracy Master / Speed Master / Attack Master / Physical Master

Natures : Jolly / Hasty / Naive / Adamant / Lonely / Naughty

How to use : Sweeper won't be able to take many hits, usually they will faint after 1-2 hits, So having the iniative is important, and just as much important is to faint the enemy monster within one hit or 2.
Damaging moves which will also restore the users HP or increase the stats are the way to go.

2.Special Sweeper :
EVs : Basicall the same as the physical one but with sp.atk instead of attack it should go like -> 252sp.atk and 252 speed and 6HP

Abilities : Accuracy Master / Speed Master / Special Master / Sp.Atk Master

Natures : Timid / Hasty / Naive / Modest / Mild / Rash.

How to use : Same as Physical.

Tank :

A tank is a pokemon with high attack and good survivability. Mainly used to deal with walls or other tanks. There are several variants of tanks i will list the 3 most commonly used.
Physical Tank :
EVs 252 Attack 252 Defense however you might want to ev less in those stats and put some in HP and speed

Natures : Adamant / Brave / Naughty

Abilities : Shield Master, Perfect Body, Accuracy Master, Attack Master, Physical master, Slow Tough, Slow Strong, Regenerator-Expert Regenerator.

How to use : A physical tank is best used against physical hitting opponents. With the survivability tanks provide, you have the luxus of increases / decreases stat's with non-damaging moves and still hit hard.
A maybe better way to counter is listed in purple below.

Special Tank :
EVs : 252 sp.atk 252.sp.def and 6HP however you might wanna ev less in those stats and increase hp or speed a bit.

Natures : Modest / Quiet / Mild

Abilities : Shield Master, Perfect Body, Accuracy Master, Special Attack Master, Special Master, Slow Tough, Slow Strong, Regenerator-Expert Regenerator.

How to use : A Special tank would come to handy if you have a special hitting opponent. With the survivability tanks provid, you can increase your damage and defenses even more with non damaging moves and decrease those of your opponent.
A different may better way to counter is listed in pruple below.

Allround Tank :
EVs : 126 Def 126 Sp.Def 252 in either Sp.Atk or Atk

Natures : Adamant / Modest / Brave / Quiet

Abilities : Shield Master, Perfect Body, Accuracy Master Special Attack Master, Attack Master, Physical Master, Special Master, Slow Tough, Slow Strong, Regenerator-Expert Regenerator.

How to use : This is the most versatile variant, it will perform a bit worse than the specialized tanks, but it can take on both.
You basically want to increase your stats and decrease the oponnents. It works more like a wall than a sweeper, because of it's slow speed, but it's able to hit hard.
So while dealing damage to the opponent, you have the luxus to increase your own stats via non-damaging moves because of the survivability a tank build provides.

On top there are Tanks which have One defense stat and a high atk stat of the oposite like High Def and High Sp.Atk with that you can counter other tanks more effectively. Because Of the builds listed aboveA tank with either high def/sp.def and a high atk stat of the other site can hit harder because those tanks won't be protected from attacks of the other site while you still have the defenses to take their hits effectively.

Wall :

A wall is a pokemon, specialized in defending, it's not meant to deal damage, but furthermore to debuff the opponent's pokemon or to put status effect on it.
EVs : 252 def 252 sp.def however, you might want to putt less in the defenses and a bit in hp if that stat is low.

Natures : Bold / Impish / Relaxed / Calm / Careful / Sassy

Abilities : Perfect Body, Regenerator / Expert Regenerator, slow tough, cautious, Radiation, Shield master, Defense Master, Sp.Defense Master, Camouflage, Effective Healing.
How to use : Basically, you need healing moves, and status effect moves, to make the enemy bleed / burn / poisoned or moves to debuff the opponent's stat. If your wall got radiation, it will do 10%damage.
Walls are the easiest and mos reliable class in the game.
2.Special Wall :
EVs : 252Sp.Def 252HP 6Def

Natures : Calm / Careful / Gentle / Sassy

Abilities : Sp.Def Master, Radiation, Regenerator / Expert Regenerator, Effective Healing, Shield Master, Camouflage, Perfect Body, Slow Tough, Cautious.

How to use : Basically the same as Walls, except that Special Walls are there to counter Special attacker like special tanks or special sweeper.
3.Physical Wall :
Evs : 252 Def 252 HP

Natures : Bold / Lax / Relaxed

Abilites : Def Master, Radiation, Regenerator / Expert Regenerator, Shield Master, Camouflage, Effective Healing, Slow Tough, Cautious.

How to use : Same as Special Walls, except that it's used to counter physical tanks or physical sweepers.

Special and Physical Walls, will have incredibly high def and sp.def, but only one of each, so don't use a Physical Wall against a Special Attacker or it will go down in a blink of an eye.

*-credits to blau in pokemonpets for his thread on this*