Hi Karen,
Given how well this site is doing in SERPS, you might want to consider
NOT adding custom description tags!
In fact Some SEO'ers will deliberately block search engines from using
the DMOZ description (by using robots.txt file) because it 'may be old
and outdated' or is otherwise not optimal.
You are correct - sometimes Google will choose to use snippets of text
from the webpage rather than the description tag. At least that's what
I've always "understood." However, I recently checked one of my own
sites to see what Google was using, and it was using the description
tag I'd written.
Given the big debate going on about whether Google will be dropping
DMOZ results, BOTW maybbe buying DMOZ, or directories going down the
tube altogether, I think I'll get a blinkin' blog post written about
it and bring it up here for discussion :-)
Fiona