What's the difference between device file and its raw version?
On OS X, the r means raw device (same as on other UNIX) and on OS X the raw file is a character based I/O and the normal diskX... is a block special device.

Both of these entry points allow a program to access a bare stream of information on the drive without interacting with the filesystem (file names, paths, etc..) and the character device bypasses any buffers (that both speed up access and slow it down depending if you are making large or small operations).

Wiki has a nice article on these general concepts: http://en.wikipedia.org/wiki/Device_file