Christian wants to know the difference between a router and a modem. Leo says that they handle two different jobs but some people get a modem that also works as a router from their ISP. Modem means "modulate-demodulate," and in the early days, it would take the bits and turns them into sound and then back again over a telephone line. Now they send the data digitally. Then it converts it into RF signals and back to bits.
Routers, by contrast, juggle multiple devices and route traffic to each individual device. It's like a switching station. Routers can also convert data into radio signals for wireless (Wi-Fi) transmission. Leo says that's why buying the modem and router separately is better than getting them from the ISP. They're newer, can be updated more often, and are more secure. You'll also save $10 a month on the rentals. But if they force you to use their modem, you can still get your own router and put the ISP's router into "bridge mode" to pass the traffic to the router.