Pokémon GO: 11 Biggest Questions Answered For New Players
9. How Much Can I Do Without Paying Any Money?

Everything!
The app/game is free on all devices, meaning Niantic had to design a fairly standard 'free-to-play' model around it. As such, everything from Pokéballs to monster-summoning incense and whatever else can be accrued by simply playing the game, walking around and flinging Pokéballs in all directions.
However, there is a shop filled with the arbitrary microtransactions, where - if you want - you can just purchase higher-level Pokéballs and other items immediately.
Real-world cash is traded for coins, which then buys these items. It's the same system you've seen in everything from Game of War to Clash Royale. It keeps the entire base product free, but puts the vast majority of us at the mercy of those with far looser purse strings.
Time will tell if gym battles end up feeling incredibly unbalanced because some peoples' Pokémon have been charged up by a ton of optional buffs and items, but right now it's a fairly level playing field.