Team Roles

As you may or may not know (you probably do), each pokémon has different stats, movepools and abilities, a combination of things that can be messed around to achieve completely different purposes. Let's take a Hippowdon and look at its stats: high HP, Defense and Attack... clearly Physically oriented but it has low Speed so we can conclude that it's made for defense purposes. In fact, HP and Defense are the most important stats to consider in a Physical defender and the high Attack gives it a chance to retaliate while walling hard hits, so we can deduct it's physical and defensively oriented. Furthermore, we can look at its movepool and see the presence of an immediate recovery move (Slack Off), something almost always exclusive to defensive pokémon. Bottomline: Hippowdon is standardly a Physical Wall. Those are the three main aspects to consider when you decide the role a pokémon will have on your team: stats, ability and movepool. You will have to set apart offensive or defensive power and Physical and Special power (alternatively considering it "Mixed"). Now let's run over the most common roles.

Sweepers

Sweepers are pokémon that count on at least an offensive stat (Attack / Sp. Attack) and usually Speed well developed along with a versatile movepool, with high base damage moves and type variety to be able to take advantage of counters' weaknesses and OHKO/2HKO them.

Physical Sweepers - Sweepers that use only Physical moves and therefore don't depend a bit on the Special Attack stat.
- Rampardos, Aerodactyl, Salamence, Mamoswine, Breloom, Garchomp, Arcanine, Rhyperior, Tyranitar, Heracross, Lucario, Infernape, Groudon, Rayquaza, Deoxys and Deoxys-A.
Special Sweepers - Sweepers that use only Special moves and therefore don't depend a bit on the Attack stat.
- Alakazam, Porygon Z, Gengar, Espeon, Magnezone, Heatran, Arcanine, Lucario, Dragonite, Salamence, Magmortar, Darkrai, Deoxys, Deoxys-A, Dialga, Kyogre, Latios, Rayquaza, Mewtwo and Palkia.
Mixed Sweepers - Sweepers with both offensive stats above average and that work with a moveset divided between Physical and Special moves.
- Azelf, Salamence, Infernape, Tyranitar, Metagross, Arcanine, Lucario, Deoxys-A, Deoxys, Mewtwo, Dialga, Palkia, Rayquaza and Ho-Oh.

Walls

Walls are pokémon that count on at least a defensive stat (Defense / Sp. Defense) and usually HP well developed along with a recovery move, barriers, status inducing moves, etc. and a couple of high base damage moves to retaliate.

Physical Wall - Walls specificaly designed to defend your team from Physical Sweepers. Much stronger in Defense than Sp. Defense and usually high HP.
- Steelix, Regirock, Registeel, Forretress, Gliscor, Skarmory, Suicune, Rhyperior, Tangrowth, Cresselia, Weezing, Hippowdon, Slowbro, Uxie, Deoxys-D, Giratina, Groudon and Lugia.
Special Wall - Walls specificaly designed to defend your team from Special Sweepers. Much stronger in Sp. Defense than Defense and usually high HP.
- Blissey, Spiritomb, Regice, Snorlax, Cresselia, Registeel, Cradily, Umbreon, Dusknoir, Milotic, Suicune, Uxie, Vaporeon, Deoxys-D, Giratina, Ho-Oh, Latias and Lugia.
Mixed Wall - Walls with both defensive stats above average and usually high HP.
- Registeel, Cresselia, Uxie, Cradily, Umbreon, Dusknoir, Bronzong, Suicune, Deoxys-D, Giratina and Lugia.
Bulky Waters - A small group of Water-type pokémon used to counter Dragon-type Sweepers. At least a high defensive stat and HP, the idea is to take a hit and OHKO with Ice Beam.
- Suicune, Swampert, Milotic, Cloyster, Slowbro, Gyarados, Vaporeon, Tentacruel and Manaphy.

Supporters

Supporters usually have a high Speed or defensive stat so they can do their job before being crushed by a strong Sweeper. They're basically pokémon that take advantage of less direct moves of their movepool to make the difference in a match.

Hazers/PHazers - Very commonly used against Baton Passers, Hazers use the move Haze to nullify all stat changes in the game. Pseudo-Hazers (PHazers) use Roar/Whirlwind to make the opponent switch, thus losing all stat changes.
- Suicune, Weezing, Vaporeon, Steelix, Hippowdon, Skarmory and Donphan.
Baton Passers - These pokémon raise their stats with moves like Dragons Dance, Calm Mind, Swords Dance, Agility, etc. and then use the move Baton Pass to switch to another pokémon while transfering all stat changes to it. It's also used to keep an opponent trapped with Mean Look or to keep a Substitute.
- Celebi, Umbreon, Ninjask, Yanmega, Vaporeon, Jolteon, Leafeon, Espeon, Scizor and Togekiss.
Clerics - Also called Healers, these pokémon use moves like Aromatherapy and Heal Bell to cure the entire team or Wish/switch out to heal specific pokémon. Leech Seed can also be used with this.
- Blissey, Miltank, Jumpluff, Meganium, Celebi and Shaymin.
Rapid Spinners - Spinners are pokémon that use the move Rapid Spin to remove Spikes/Stealth Rock/Toxic Spikes/Leech Seed.
- Starmie, Forretress, Cloyster, Claydol, Donphan, Blastoise and Hitmontop.
Spikers - Spikers use the moves Spikes/Stealth Rock/Toxic Spikes to damage opponent pokémon switching in.
- Skarmory, Cloyster, Forretress, Kabutops, Donphan, Bronzong, Hippowdon, Rhyperior, Mamoswine and Jirachi.