What's a Pokémon Go gym and how do I master them?

Everything you need to know but were afraid to ask about Poki(C)mon Go gyms, battles and how to win them

Half of Poki(C)mon Go is collecting the little beasts, you've got to "catch 'em all!" after all, but the other half is a little more daunting: gym battles. But they needn't be overwhelming or scary. Here's a complete guide to what gyms are, how to battle in them and, most importantly, how to win.