The site where i'm at the moment has a nice example.
They have here a data network of there own and we provided the voice part.
Vlan 1 (default) is used for the cusomers data lan.
Vlan200 is used for our voice network.
Now a port on switch where a pc and a phone are connected looks as following:
interface FastEthernet0/3
switchport mode access
switchport voice vlan 200
Then there is also a port connected to a switch of their own which has their own dhcp server on it. This port is set in trunking mode wich allows all vlan's
Then there is a second port that is connected to our voice network. This port is also a trunk but only allows vlan 200.
What happens when a cisco phone is plugged in to port 3 is than the switch recognises the phone (cisco-cisco

) and automaticly send all the traffic of the phone to vlan200.
Which goes to the trunk with vlan 200 and here it finds the dhcp server of the voice network.And so it is getting an ip address from within the voice network.
On the otherhand when a pc is connected the switch will see this as a "not voice" device and will put all the traffic of this one in vlan 1. And goes out on the trunk port that is connected to the part where their own dhcp server is
And this way it gets a address from their own dhcp server.
I hope this clarrifies you question