Google Voice is a service where you are assigned a phone number that can make all of your phones ring (landline/cellphone/etc.). Voicemails are sent to your e-mail and free calls and text messages can be made within the U.S. and Canada. To get access to Google Voice, you needed an invitation, but now Google is offering the service for free. However Google Voice is limited to everyone in the U.S. for right now. Below is a video about what Google Voice does.
[Google Voice Blog]