A modem is a device that modulates data into an appropriate transmission medium. Modems are required to convert incoming analog connections (i.e. from your Internet Service Provider) into digital data.
Similar in nature, a router has the ability to split and distribute data connectivity to multiple devices. Routers usually vary from 4 to 8 ports, but can be upto 48 ports.
There are modems with inbuilt routers, which is very common. Allowing you to easily connect to the internet, and provide connectivity to your home network within one device.
Modems are usually classified by their speed rating, which is commonly bits per second. The higher the rating, the faster the connection.
When selecting a modem/router, pay attention to the wireless capability, the speed, the type (i.e. VDSL modem). These often last years and you want to get it right.
If you're unsure what type of internet connection you have, simply ask your ISP, then call JW Computers, we'll be able to help you select the right modem or router for your needs.