It's a call forwarding system mostly. You can redirect your calls from your GV number to your cellphone or your home phone. Same thing for text messages.
Yes, you can call anywhere for free in US and Canada using Google Voice in your computer. Just go to gmail and under contacts at the left side, there is a call phone where you can call for free. Other people can call you but the phone won't ring unless you have forwarded it to your cellphone or home phone.
You can also send and receive text messages for free in pc and in your phone (must download an app for the phone).