As the name implies, hosts offering unmetered servers do not charge based on the amount of bandwidth consumed. Rather, the customer pays a flat fee per month. This does not mean the customer has unlimited data transfer, however. This is limited by the speed of the connection.
Most unmetered servers share a connection (often 100 mbit) with other servers. This makes the service more affordable, but limits the amount of available bandwidth. Think of it in terms of an amusement park. You can ride anything you want once you enter, but must share the rides with other customers.
For those with extremely intensive needs, dedicated lines can be purchased. This is akin to renting the amusement park for yourself. While you get all the resources 24/7, this luxury comes at a heavy price– often many times the price of a server. Typical speeds available include 5 mbit, 10 mbit, 100 mbit, and even 1 gbit!