JayMatt – Google Voice is a ‘one-stop’ solution for people, giving them one phone number (based in the US only, at the moment), and allowing multiple physical phones to ring. This is useful if you do not want to give out home/mobile numbers etc – Google Voice will allow you to set up the phones to ring when people phone your Google Voice number.
Other features includes call screening, being able to listen in before picking up, sms sending/receiving, call forwarding, etc
It’s based on a technology pioneered by a company called GrandCentral, which Google took over some time ago – this has been eagerly looked forward to as a new way to send/receive communications
Hope that this helps!