There are 3 primary ways Google gets your description tag for the search results.
The first being the meta description tag that is in the header of the code. This will show up most of the time if the keyword being searched for is in that tag (which is why we put the primary keyword first in the description tag in the header).
The second way Google builds the description tag is generating it based on the search term. Google goes through the on page content to find the keyword you searched for, and pieces together the keyword, and a word or two on either side of it, and uses this mash-up as the description tag in the search results. You can tell when this technique is used because the description tag does not read fluently and looks like random sentences pieced together.
The third way that Google generates a description tag, which is used least out of the 3, is from entries for the specific page in dmoz or the yahoo directory. This is why it is important to keep that entry up to date, just in case it needs to be used.
The META description is a de-duplication cue, and removing it will put your snippet even more under Google’s control (although having one doesn’t stop them). Dr. Pete