A tablet is bigger than a smart phone, smaller than a laptop, and has no physical (built-in) keyboard.
- iPad and iPad Mini are Apple's tablets
- Android has many different tables under many different names (varies by manufacturer)
- Amazon's Kindle Fire is a tablet (also Android-based, but that fact is downplayed)
- Microsoft's Surface is a tablet (with optional physical keyboard add-on) that is made by several manufacturers.
As for which one to get, that's really a matter of preference. What does your son want to do with it? What do his friends have?
I recommend you get an operating system that will be in line with your peers and preferences. Does your son already have an iPhone? If so, an iPad may be the best choice because most of the apps will also run on the iPad. Same scenario for Android phone/tablet. Are he and his friends Windows 8 users? If so, you may want to consider a Surface.