What's the difference between device file and its raw version? It's convenient to use dd to write image file to USB sticks. But I find sometimes people write dd of=/dev/disk1 and there is alos dd of=/dev/rdisk1. 

e.g. 


The arch wiki



 dd if=image.iso of=/dev/disk1 bs=8192



The Raspberry Pi instruction



sudo dd bs=1m if=~/Downloads/2012-10-28-wheezy-raspbian/2012-10-28-wheezy-raspbian.img of=/dev/rdisk3


What is the difference?
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