Pokestops are scattered all over the game world. Some areas will have more than others depending on the area that you are living in and how many areas of interest there are. A pokestop will typically be placed on top of some kind of landmark or location of interest to mark the location.
When you get to a Pokestop you can tap the screen to open up the information about the location. This will display a text box at the top with some information about the location and a circle below with a picture. If you swipe across this picture it will spin and allow you to harvest the items that are available in this pokestop.
How Long Does It Take For Pokestops To Refresh
Typically it takes 5 minutes or so for the location to refresh, allowing you to harvest it again. When you harvest a Pokestop it will turn purple and the icon on the map will also be purple. After 5 minutes the location will begin to turn blue again. Once it is solid blue you will be able to harvest the Pokestop again and collect the items that it drops.