The Basics

To succeed in competitive battling you need to know and do certain things some of these (just naming a slight few of the many) are how your team works together and the threats that are in the metagame and how to respond to them. If you don't know this you will then you'll fail. So what is a Pokemon team made up of? Well that's simple, Pokemon. But in cb they are more tan just any odd pokemon that you find in victory road... they are hand selected and bred battling machines built to win. In a Pokemon team you can have up to six members, each one of these members has a specific role to play and if they do not fulfill it you will fail =( .

The jobs of pokemon:
I will list the general roles that each pokemon can play and then write some information below them to explain.

This is a Pokemon that sweeps through the opponents team. They are designed to take out as much of the foes team as possible before being KOed or until all 6 of the opponent’s team are gone. There are 3 types of Sweeper (physical, special, mixed).

Examples of Sweepers:
Special: Gengar, Starmie, Porygon-Z, Alakazam.
Physical: Dragonite, Garchomp, Rhyperior, Heracross, Weavile, Metagross
Mixed: Swampert, Infernape

If you don't already know what physical, special or mixed mean don't worry. It is very simple, all it means is the type of attack the pokemon will use. Physical attacks are the ones that make contact for example, brickbreak would be classed as a physical move as it makes contact with the opponent. Special attacks are attacks that inflict damage without contact a good example would be Psychic. When choosing moves for your sweeper look at it's stats and see which out of Attack and Sp.Attack. If the Pokemon has a higher Attack stat that special attack select physical moves as they will do more damage. If the Pokemon has higher Sp attack choose Special attacks as they will do more damage. In the case of the Pokemon having the Attack and Sp Attack of ruffly the same level you could choose either physical, special or a mix of them.

This is a Pokemon that stops sweepers taking your team apart. Believe me, this new generation  (Diamond, Pearl and the upcoming Platinum) is focused on being the hardest and fastest, so you'll need some big bulky pokemon  here to survive.

Examples of Walls:
Special: Blissey, Cresselia, Milotic
Physical: Skarmory, Swampert, Rhyperior
Mixed:   Suicune, Milotic*

Walls are basically a pokemon that is meant to stop the Sweepers or any other attacking pokemon from damaging you team, they keep out all those unwanted attackers such as Garchomp, Gengar and Weavile.
Like sweepers, there are physical, special and mixed walls. The
main characteristics of a wall are High Def if it is a Physical wall or high Sp.Def if it is a Sp.Wall. If it has both high defense and high Sp.Def it is considered a Mixed Wall. All walls also have high HP (hit points, health points what ever you want to call them).

* Milotic has great Special defense and pretty good def. Though it is mostly used as a Sp.Wall it can be used as a Mix.Wall and a Tank (I will explain tanks later).

The Sweeper[s] and Wall[s] of your team are the most recurring jobs of a pokemon in cb (competitive battling)
but they aren't the only uses pokemon can be in competitive battling. There are many more though at this stage in the Competitive Battling Tutorial I will only mention the most used.