What are Pokemon Go Gyms for?
Pokemon Go gyms are where you battle or train your Pokemon. Each Gym is owned by a team and once your Trainer reaches level 5, you’ll be asked to choose a team.
There are three teams: Instinct, Mystic and Valor. They each have a colour: yellow, blue and red, respectively. These colours are useful, because you can glance at a map and see which Gyms are friendly, and which aren’t.
Ultimately the reason to be in a Gym is to get the PokeCoins which you can use to buy stuff in the Store to advance in the game.
Source from:http://www.pocket-lint.com/news/138278-pokemon-go-gym-tips-how-to-battle-train-and-win
Gyms Guide
Attack and Defend Gyms
If we are lucky enough, we may come across an unoccupied Gym marked by white. In this case, we can click the icon in the lower left corner to put our Pokemon in the Gym in order to claim the Gym. Subsequently, the Gym will be marked the same color of our team. The Pokemon in this Gym will appear above the Gym. Please note that only Pokemon with the max HP can be put into the Gym for the purpose of defending.
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Attack Moves
Once players enter the Gym battle interface, the background will become the battlefield. When the countdown ends, players can attack the Pokemon that are used to defend the Gym. The energy bar will be constantly filled. Once the energy bar is full, players can press the phone screen to use Pokemon’s unique attack moves. When the charge is completed, the moves will be automatically used. Players can also slide left or right to dodge. If the timing is proper, a dodge message will pop up, and Pokemon won’t be hurt.
Super Effective and Not So Effective
When Pokemon perform attacks of certain types or elements or get attacked by certain types, if the damage is effective, then it will double the damage. A Super Effective message will pop up. If the damage Pokemon receive is just average, the damage will only be half of the original one, and a Not So Effective message will pop up.
Source from: http://www.pokeaccountbuy.com/news/10–pokemon-go-guide-to-gym-and-how-to-join-a-team
Strengthen Your Pokémon
Before even considering gym battles, you first need to power up and evolve several of your Pokémon to ensure they are strong enough to compete. If you have been capturing loads of Pokémon up to this point, you should have enough Candy and Stardust available to strengthen at least a few Pokémon.
Your Pokémon’s strength is based on its CP, or Combat Points. You can Power Up a Pokémon to improve its stats even further. Power Ups require both Stardust and Candy, which can be accumulated by capturing and transferring Pokémon. In general, you will want to transfer low CP Pokémon of a particular species back to Professor Willow in exchange for Candy. Save power ups for Pokémon with naturally high CP in order to maximize the stat increase.
Evolving Pokémon adds another layer to the process. An evolved Pokémon will have better stats than its previous form, but it will likely take on a completely new move set. If you intend to evolve a particular Pokémon, you should do this first before powering them up, just so you can see the stats you have to work with.
Source from:http://www.usgamer.net/articles/pokmon-go-gym-battle-guide-how-to-claim-gyms-and-win-battles-10