A control surface is a piece of hardware which hooks up to your computer and allows you to control various track parameters, such as volume and pan. Many control surfaces also provide buttons for controlling recording, at the very least with play, record, stop, and seek, though even better ones come with assignable soft buttons which will allow you to do fairly complex stuff in your DAW.

Most control surfaces communicate via MIDI, but some newer ones hook up via USB (though usually they internally use a USB-connected MIDI interface). Some are also integrated into other pieces of hardware, such as with most of Tascam's A/D boxes.