Now, to start off, I should mention the basic set up for a competitive team. Generally, you have your lead, your walls, your sweepers, your supports, your revenge killer, and the less common hazer. In this lesson I will teach you all there is to know about a sweeper.
What is it?
A sweeper is your power house pokemon. This is the one you will use to "Sweep" the enemy team. Sweeping is simply knocking out the enemy pokemon with ease, typically with one hit. In most battles it's the sweeper that determines who will win. In most cases, though, sweepers aren't very bulky, and are easily taken down if not played right. Another thing to worry about is being walled by enemy pokemon, or being stopped mid sweep.
Important to note
It's typically a smart move to have more then one sweeper on your team, but you don't want to have too many. I generally limit myself at three, and even that is a rarity. By having extra sweepers on your team you won't be out of the battle just because your sweeper gets KO'd.
Item: Life orb
Ev's: 252 Atk/6 Def/252 Spe
Moveset: Dragon Dance
Possessing a powerful moveset while packing in nice physical attack and speed, while carrying some good resistances, Gyarados is able to sweep many unprepared teams with ease, and utilizing Dragon dance to gain even more speed and attack makes him an excellent choice for any team.
Item: Life orb(I'll explain item choices for pokemon in a later lesson)
EV's: 4 Def/252 spA/252 Spe
Moveset: Fire Blast
Hidden Power Grass
Charizard, being a fairly under rated sweeper, can pull off some fantastic sweeps, using his nice special attack to wipe out the competition. The only issue with Charizard is his typing. Being Fire/Flying, Rock type moves are 4X super effective, and stealth rock will take down half your health upon entry, making it fairly difficult to sweep without setting up fo him with the rest of your team.
The sweepers are easily one of the most important components to your team, and essential if you want any hope of winning. I have given an example of a physical sweeper and a special sweeper, but thee are so many I couldn't even list them let alone give sets for them. It's always important to remember your sweepers limits, and as tie goes on you will get the hang of different pokemon's speeds. For more information and some expert sets, I suggest going to http://www.smogon.com/
Next Team building Lesson: Walls