Skull One is mostly correct. If you have both an iPhone and iPad signed into the same iCloud account you can send and receive at your iPhone's number, but otherwise you have to use your email address. By default your Apple ID is the only address configured to receive iMessages on an iPad otherwise. If you go to Settings > iMessage and tap Receive At.. you can see a list of Address(es) and (if you have an iPhone using the same iCloud account) phone number your iPad can receive messages at. Only addresses with a check mark by them can actually receive messages on the device.