Messages only works wth other iOS devices - iPads, iPhones and iPod Touch devices. You can also message with a Mac that is running Mountain Lion.
Before you can message another user, they have to set up messages on their own devices. You cannot reach them in a text, if they have not set it up yet themselves.
You can text to an email address or an iPhones phone number that is associated with the Apple ID of the person that you want to reach. You can use a phone number on your own devices as the "contact at/send and receive at" for messages as long as it is an iPhone number associated with your Apple ID.
iOS: Using Messages - Support - Apple
iOS 6 and OS X Mountain Lion: Link your phone number and Apple ...