Its battling. One person fights another, it being on the bus, over Wi-fi, or anywhere else, really, but usually on Wi-fi.
You should know that hacked Pokemon are STRICTLY not allowed. WE HAVE HACK CHECKS!
Anyway, you must learn the ranks of Pokemon, "tiers" or "environments:
(U) Uber - These Pokemon are too devastating to be used in normal play. These Pokemon are banned from Standard play unless someone asks for an Uber battle, then these are okay. Here is a list of Uber Pokemon:
Deyoxys (All Formes)
Giratina (All Formes)
Sudowudo and Wobbuffet are on there because they have high stats and great movepools and abilities. Latios, Latias, Jirachi, and the trios (Raikou, Entei, Suicune, Zapdos, Articuno, Moltres, etc.) are allowed.
(OU) Overused (Standard) - Pokemon that are commonly used, this is actually a huge list, it's about 70% of the Pokemon. When people ask for a standard battle, OU, UU, NU, and LC are allowed.
(UU) Underused - Pokemon that are not often used, like Azumaril, are in this category. When someone asks for an Underused battle, you can use UU, NU, and LC Pokemon.
(NU) Neverused - Pokemon that are almost unusable like Magikarp go here. It's not often that someone will ask for this, but only NU Pokemon are allowed here.
(LC) Little Cup - One of the most fun tiers, you use Pokemon that are still able to evolve and under 3 tall. So, for example, Duosion is allowed because it evolves into Reuniclus and is 3 feet tall. Gothorita is not allowed because, although it evolves into Gothitelle, it is 3 feet 4 inches, thus exceeding the height limit. Wynaut is not allowed, it's an uber.
Well first, pick 6 Pokemon. Check out the Pokemon Analysis pages (coming soon) for some good Pokemon and sets. Hold up, sets? Yes, each Pokemon needs a set to function correctly. A set consists of Set Name, Pokemon Name, Item, Ability, Nature, Moves, and EVs. Check the respective guides to find out more about these topics. So, as an example, I'll be using my favourite Pokemon, Whimsicott. Here's a really great set:
Set Name: SubSeeder
Pokemon Name: Whimsicott
Item: Leftovers - Increase HP on every turn.
Nature: Calm (+SpDef, -Atk)
Energy Ball - Pretty much a filler move.
Cotton Guard - To increase defense, makes your Sub last longer.
Leech Seed - To drain HP while defending with Sub.
Substitute - Defend while draining HP with Leech Seed.
EVs: 252 SpDef, 252 HP, 4 SpAtk. The SpDef & HP can help cover what Cotton Guard doesn't. The SpAtk is for Energy Ball.
See? Nice set, right?