From Francois Dupoux:
Partition Image for Linux saves partitions in the ext2fs (the Linux standard), hpfs (the OS/2 file system), ReiserFS (a new journalized and powerful file system), or FAT16/32 (DOS and Windows file systems) file system format to an image file. The image file can be compressed using GZIP/BZIP2 to save disk space, and split into multiple files to be copied onto floppies. Partition Image copies only used data from the partition. Free blocks are not written in the image file to enhance speed and reduce image size. If you save a big and full partition, the image can be big as well (the size of an image of a full 1GB partition compressed with GZIP can be 400MB).