The 'drive imaging' method of backing up data involves making an identical copy of a partition (a grouping of some or all of the space on a hard drive so that the operating system can access it as a logical drive like c:) and storing it elsewhere.
Generally a separate physical hard drive is used, or some sort of removable storage such as CDs, DVDs or tapes. Generally this 'image file' is compressed, so it takes up considerably less space than the original. In the event of disaster, this file can be restored to a new hard drive, and for all intents and purposes, will be completely identical to the state of the old drive at the point the image was made. Very handy.
Drive imaging originated with products like Symantec's Norton Ghost, intended to make the installing and configuration of large numbers of identical PCs less of a chore. A technician would install the operating system and required programs on one computer, make sure everything was functioning correctly, then create an image file of that system which would be stored on a server computer.