Google Voice is a service that gives you a single phone number for all of your phones. You can select a phone number from Google (hopefully in the same area code as you are), and use this number to take advantage of advanced services such as: Voicemail to email, voicemail transcribing, call conferencing, international calls, blocking and screening. Never miss a call again and my favorite use is that I never have to give someone my actual cell number again!
Once you have signed up login to your google account and setup your google voice account. It will ask you to select a phone number and authorize a phone you would like to use with the service. From the browser based control panel you can add phones, setup voicemail, view and listen to voicemail and manage your google contacts.
Contacts can be setup into groups and either full groups or individuals can be blocked, screened or allowed to ring through. Multiple phones can be used with the service. For example I have my cell, work and home phones all synced to the google voice system. When someone calls my google voice number, all phones will ring, or not, depending on how I have the system setup.
To use your Google Voice account with your cell, home or work phones you will need to dial a number to access the google system and then initiate your call to the intended party. When the call is placed using the Google system, your Google Voice number is displayed to the recipient and not your actual cell number. This option however is configurable via the Google Voice control panel. Applications for cell phones that automate this process are available for Blackberry, Palm, Windows Mobile and Android devices. Android phones obviously have the best solution for this as it is written by Google.
Google Voice, 1 number for everything!