If you also have an iPhone you can send and receive standard SMS and MMS messages from your iPad instead of just iMessages. To do this will need to ensure that both your iPhone and iPad are running iOS 8.1 or above, and logged into the same iCloud account on the same Wi-Fi network.
To enable SMS/MMS messaging open Settings on your iPhone, then select Messages and ensure the iMessage option is enabled. Then select Text Message Forwarding and turn on the option for forwarding to your iPad. Ensure your iPad is unlocked and in a short moment your iPad will popup with an authorization code which you will need to enter into your iPhone. You will then be able to send an receive SMS and MMS messages from your iPad.