Why can't I send or receive text messages on my I pad.

1. Messages on the iPad can only be sent to other iDevices

2. They must also be registered with iMessage

Note: The two above conditions must be met.

Adding to Diavonex' explanation:

Why can't you send or receive text messages? Because the iPad is not a telephone.

You cannot send text messages however you can send iMessages to iPhones, iPads, iPod touches and Macs that have the iMessage feature enabled. You need internet connection to send iMessages if you are not connected to WiFi or 3G or 4G this feature will function.