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.
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.
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.