What is the difference between an Atlas and a Parcellation? I come from a computer science background and am just starting out in the area of analyzing brain imaging data (fMRI predominantly) and while going through some of the literature, I am kind of confused about the difference between a brain atlas and a brain parcellation? 

Are the two used interchangeably? Does one correspond to anatomical units and the other to functional units?
[This is an older post, but...]  A brain atlas is going to, typically, be a set of images or volume that is labeled anatomically or functionally.  Typically one would take a single volume of data and register or align it to the atlas (or vice-versa).

Parcellation is a way of segmenting sub-regions of a particular region of the brain. (What a "region" is or "sub-region" might be a bit up for debate).  

One could do image registration / alignment of an MRI data volume to an atlas (that is parcellated or segmented) and then the parcellation can be applied to the MRI data volume.