After playing the game for a little while, I've noticed that Pokestops are chosen seemingly at random locations on the map. A few of them are even in inaccessible places; an abandoned church building, inside of an airforce base, etc. How are these locations chosen as Pokestops?
Pokémon GO's database is based on Niantic's portal database for Ingress, which is currently not accepting new requests for more locations while they review the process.
If you want to see every current PokéStop, download and register for Ingress, then go to https://www.ingress.com/intel
Keep in mind that, while every PokéStop and Gym are based on Ingress Portals, not every Portal will be a PokéStop or Gym.