AOL and Yahoo use Google for their search results, so my answer
applies to all three.
One reason could be that the site is using Flash. This Google search
results page for [flash intro] has a few sites without descriptions,
and one without a title as well:
Using frames without a title tag can also be the cause - look at the
results at the bottom of this Google search results page:
Page titles can only appear if the page has a title. Most do, but
there is nothing stopping a webmaster from not titling a page.
Descriptions are snippets of text found on the page - if there are no
words to index, then there will be no description. Google cannot index
words within Flash or image files. A site made up entirely of images
does not have any text to index.
It can also occur because Google is aware of the page in question but
has not indexed it. Basically it sees a link to a page, but does not
index it because:
A) A robots.txt file from the site tells Google not to index it. See
Jill Whalen's thoughts at HighRankings.com:
B) Google chooses not to - usually because the site is too big to
index everything, often because pages are dynamically generated.
The issue has been discussed at WebMasterWorld:
..the second discussion mentions that having two title tags can cause
Google to display neither. It's a bit hard to find an example.