I think of it as this:
You have one VLAN for data (VLAN 1 for example) and one VLAN for voice (VLAN 2 for example).

Many providers/installers like to separate this voice and data traffic, and it is definitely recommended on large deployments. If there are separate drops for voice and data, it is easy to segregate your traffic. However many times, there may only be one data drop available, which is where the VLANS come into play. To pass more than one network down a wire/drop, at least one (or both) must be tagged. So oftentimes, the voice traffic will be tagged and the data traffic left untagged. Then when a user plugs in a phone, the network (usually through the use of the LLDP protocol) will tell the phone as it is booting up, that its voice traffic is on tagged VLAN 2, and to simply pass untagged (data) traffic thru.