What's the difference between 2.4 ghz and 5 ghz?

We recently got a new router, and it's got both 2.4 and 5 ghz capabilities. I don't know too much about routers: just enough to set them up. What's the difference between the two? If it helps, we'll be connecting iPhones, laptops, and TiVos.

Different frequencies, so they work in a different spectrum, the 2.4 is very common in multiple devices such as cordless phones, microwaves etc. So its usually crowded with interference and neighboring routers.
you can set both up with different names and passwords, scan the the existing networks and if you see the 5g, connect to that one, that would ease the traffic for devices that are not capable to work with the 5g and have to connect to the 2.4 g, and it ll be better for the first ones.