PokéStops can be found throughout cities, though usually concentrated in higher population areas. They're generally attached to historic monuments. They show up on the map as blue cubes. Once you close enough to a PokéStop, it will change to a circle and you can use it.
Click a PokéStop to zoom in on it, then swipe to the left or right to spin the center circle. It will release a few items for you to collect. You can either click each bubble to pop it and collect the item, or close the PokéStop which will automatically collect all items.
Once you have used a PokéStop, it will turn purple and cannot be re-used until it refreshes and turns blue again, about 5 minutes later.
10 PokéStop bonus[edit]
If you spin 10 unique PokéStops in a row, the 10th stop will grant additional items and 100XP bonus. A PokéStop will only count towards the bonus if it has not been spun within the past 15 minutes. If you wait more than 10 minutes between spinning two PokéStops, the count will reset.
Lure Module[edit]
Once you acquire a Lure Module, it can be attached to a PokéStop. This increases the frequency of Pokémon that will spawn nearby. This affects the PokéStop directly, and will benefit all nearby players. It will show up on the map as a shower of pink petals around the PokéStop. Only one Lure Module can be attached to a PokéStop at a time, but multiple nearby PokéStops can have Lure Modules attached so their effects overlap.