If you are on iOS 8 on both the iPhone and iPad, and using the same iCloud account, you can go into the iPhone's Messages settings and turn on Text Message Forwarding. This will enable your iPad to use the iPhone's SMS texting for both sending and receiving texts.
Be sure Send as SMS is also turned on for the iPhone.
The iPad by itself can only send iMessages, which require that the receiver have an Apple device or computer. SMS, which is a cellphone service (not from Apple) can send and receive from almost any phone. When the iPhone decides that the number you are sending to can not be reached using iMessages, it automatically sends it using SMS instead (if it's turned on).
Obviously both the iPad and iPhone must have an internet connection for this to work.
Hold in mind that your cellular provider may charge your if your SMS plan does not cover the cost of the SMS texts.