Traditional backup works by accessing the file system of the host operating system in the usual way to read and write files when backing up and restoring. This is the most elegant way of achieving the task, but has the disadvantage of being slow. Since the files you want to back up have to pass through a separate process, the delay in accessing them is greater. RAIDs will guard against disk failures, but deleted files, whether accidentally or maliciously, still need to be recovered.
Image backup is a way of protecting files in a short amount of time.
In order to perform backups faster, the file system is bypassed and the data read directly from the disk without regard to file structure. This uses the disk driver, rather than the file system, and accesses the disk in the same way as the operating system. The data is backed up sector by sector, rather than file by file. If the operating system compresses it, then it's stored in that format, rather than uncompressing it first. Fewer processes are used so the procedure is quicker.