Pokestops are shaped like floating cube when you are far from them. Once you get close, it changes to Pokeball-shaped.
Pokestop Item Chart
Items you can get at a Pokestop varies based on your trainer level. Better items can be found as your trainer level increases. Here is a chart showing items and trainer level.
Item
Trainer Level
Pokeball
Level 1
Pokemon Egg
Level 1
Potion
Level 5
Revive
Level 5
Super Potion
Level 10
Great Ball
Level 12
Hyper Potion
Level 15
Ultra Ball
Level 20
Max Potion
Level 25
Max Revive
Level 30
Pokestop Tips
1. You can spin same Pokestop by 5minutes.
Once you spinned a Pokestop, the color is turned from blue to purple. It changes back to blue 5 minutes later, and you can gain XP and items again.
2. You can use Lure Module at a Pokestop
You can put Lure Module to a Pokestop. The Pokestop starts to attract Pokemon around of it, and you can have more chances to capture Pokemon.
Lure Module
A module that attracts Pokemon to a PokeStop for 30 minutes. Other people around the Pokestop can also beneftit from the effect.
3. 10th PokeStop Hack
You will get 6+ items and 100 XP from every 10th Pokéstop as long as you keep hacking a chain of different stops. You will also get an egg if you have an empty slot available. This not a hack or against TOS. It is simply an observed pattern of hacking.