Some audio interfaces are built with the iPad in mind, others are designed to run on all sorts of systems. Apple’s audio driver technology has made it very easy for manufacturers to make their interfaces iOS compatible. iOS compatible interfaces will be labeled as “class compliant.” However, the changing nature of the lightning connector means you’ll probably need an adapter to get the USB cable from the interface into the iPad.
The interfaces designed for iOS will have the proprietary lightning cable in the box. Although if you are running an older iPad with the 30-pin connector you will have to find another $40 for a Lightning-to-30 pin adapter.
Connecting most of these adapters and cables has the annoying result of preventing you from charging your iPad. However, there are some adapters, such as the Lightning to USB 3.0 Camera Adapter that do let you plug your charger in as well (a factor worth considering if you do long recording sessions). iPads have a very limited ability to power other peripherals so only very simple interfaces will be bus powered. Most require their own power supply.