If you are running Cisco IP Telephony on your network you will most likely be using the Voice VLAN feature of your Catalyst switches. What this allows is for the use of one port on your switch to connect an IP Phone and a PC through your IP Phone.

Configuration is pretty simple as show below where voice vlan is 200 and vlan that our PC will connect to is 100.

interface FastEthernet0/1
 switchport access vlan 200
 switchport voice vlan 100

Note: When you enable the voice vlan on a port spanning-tree it is automatically set to port fast.

If you want to learn more about the Voice VLAN the following are some good references.

Configuring the Voice VLAN
Good Explanation of the Voice VLAN