Apple iPhones (and other iDevices and Macintosh computers) have a feature called iMessage. If you want to use iMessage you need to enable it in your iPhone settings.
When messages are exchanged between iMessage users they will automatically travel over the data channel. Message exchanges involving a party without iMessage enabled will travel as regular SMS/MMS.
The switch from SMS/MMS to the data channel is handled by the iMessage system automatically in the background. The user doesn’t need to change the way they use the Messages app.
Using the data channel avoids SMS/MMS fees. Also, an iMessage user who is on wifi or using a different SIM card when traveling abroad can continue messaging with other iMessage users from the Messages app using their regular phone number.
In addition to iPhones, iMessage works on other iDevices and Macintosh computers. Apple needed a way of identifying iMessage users on these devices independent of their phone number. They chose email addresses for this purpose. Unfortunately this has added some complexity to the iMessage configuration process.
If you want to set up iMessage on multiple devices you should set up the iPhone first. Here is a tutorial for configuring iMessage on an iPhone with the system software updated to iOS version 8.1.1.
Enable iMessage, wait a minute for it to activate. If you want to use iMessage on multiple devices you need to sign in to iMessage with your Apple ID. If you only want to use iMessage on your iPhone you don’t need your Apple ID and the process is straightforward.
Other options can be set to taste. If you’re not using your Apple ID then you’re all done. If you’re using an Apple ID there are a few more things to configure. Tap on Send & Receive.
This screen allows you to nominate your phone number and/or email address for receiving iMessages. There should be a tick next to only your phone number unless you need to receive messages from iMessage users who don’t know your phone number. If you don’t need to nominate your email address then you’re all done. If you’re nominating your email address tap on it so that there is a tick next to both the phone number and email address.
The START NEW CONVERSATIONS FROM setting is where you nominate either your phone number or email address as the sending address for new messages. Make sure to tick your phone number and not your email address. Congratulations, everything is configured and ready to use.
Only use your email address as the sending address if you have a specific reason to do so. Be aware that recipients who don’t already have your email address in their address book will see inbound messages identified by an email address and not your name. If your email address is obscure then the recipient may not know who the message is from. Also, the recipient will now know your email address and may add it to their address book and start sending email to it.
After configuring iMessage on your iPhone you can configure other iDevices and Macintosh computers to also send and receive iMessages using your regular phone number. On the other iDevices enable iMessage, sign in to iMessage with your Apple ID, and then configure the Send & Receive settings to match those on your iPhone. iMessage settings for a Macintosh computer are presented to you when you open Messages.app for the first time.
With iMessage configured on multiple devices you will receive iMessages on all devices and be able to send iMessages from any device.