Explaining Google Voice to someone who has never heard of the idea is no easy task.  It's more than just one phone number that rings all your phones — it provides free SMS messaging, free voice calls, incredibly cheap international calls, and even more that there's not room to list.  Google added a YouTube overview that highlights the major features, providing a strong foundation from which to expand.  Unfortunately, Google Voice is invite-only so if you're more than ready to move yet another service to Google, you'll have to wait your turn.  Hopefully they get around to opening Voice for all as some of us have been waiting for more than a few months for our invites.