Your students have the "Student" Socrative app available in Self Service. You can download the "Teacher" Socrative app via the App Store. Students and teachers can also access Socrative via Safari on a computer or iPad. Students simply visit m.socrative.com. Teachers visit socrative.com and click on Teacher Log In.
Here's a "best practices" video of Jennifer Salmon using Socrative in her Spanish classroom:
And here's an introductory video from Socrative's website:
If you're interested in using Socrative, please don't hesitate to ask for my assistance!