When in the game map, there will often be various blue portals dotted around. These are PokéStops. PokéStops are small areas based on various landmarks in the region such as monuments, town halls, works of art, churches and so forth. PokéStops don't really offer much, but you can spin the landmark on your screen in order to get between 3 and 9 items. These items appear on the screen and can be touched to redeem. You will also receive 50 XP for each PokéStop you visit, or 100 XP if you get over 6 items.
After this, the PokéStop becomes unusable for a short period of time of about 5 minutes. After that period is over, you can return to the PokéStop and get more items.
You can also assign items to a PokéStop such as a Lure Modulator. The Lure Modulator will attract wild Pokémon to the PokéStop for you and all other players in the local viscinity.
As of Version 1.17, you could start getting daily streaks of PokéStops. These increase the amount of items you obtain at the first PokéStop of the day and continue onto a streak of up to 7. When you hit Day 7, you get well over 15 items. As of Version 1.2.7 this includes evolution items as a high probability on Day 7.