So if it’s someone you want to always get a hold of you, when she dials the GV number, it can ring your mobile phone, your home landline, and your work phone.

If it’s someone you like but don’t always need a line to, when she dials the GV number, it can ring only your landline.

If it’s someone you sometimes have to deal with but don’t particularly like, when she dials the GV number, it can go straight to voicemail.

If it’s someone you don’t like, and she dials the GV number, she’ll be blocked.

A GV number is not a real phone line, but it requires a real phone line if you want to have a conversation (an internet connection alone isn’t enough, which is why it’s not VoIP). Think of it as a placeholder and gatekeeper to your real phones.

It’s kind of like how http://www.google.com is a placeholder for 74.125.155.132. What if Google changed IP addresses? Well, it wouldn’t matter, because people don’t type 74.125.155.132 in the address bar of Firefox. They type google.com.

Same deal with a Google Voice number. People dial it to get to your real phone, but if you change phone numbers, it won’t matter, because you can configure your GV number to redirect to your new number.

I think this one-minute YouTube video explains the basics a bit better than I can:
http://www.youtube.com/watch?v=m4Q9MJdT5Ds