Newbie - Why use Vlans / Qos

[David Patel_2]

Hi Guys
I'm new relatively new to HP procurve switches. Basically, I understand on typical sme/smb networks (20-25 PCs, a few servers, typical MS servers) people have switches such as 2524's or 2848s but I need to understand from a technical background why would one use Vlans in this senario or Qos and above all how. Most typical configs include pcs, servers and one of these servers is typically a DHCP server.
Can anyone give me pointers. ?
My next question is that if I have a IP enabled Nortel BCM Voice system on the same type of network and want to use IP Phones (nortel), I know we can plug them in to the Switches (assuming the handsets are powered through mains adaptors), set them to dhcp and point them to the main Voice BCM system and they will work as phones being able to make and receive calls reliably internally as well as externally, but where / why and at what point do I need to get Qos or Vlans involved in this senario?

Sorry for such a vague set of questions; I'm trying to get to understand the background but in real life exmaples.?