I assumed it was strictly by the meta 'description' tag. But I noticed that SO doesn't use this tag and has descriptions for its links in google search.
I have not yet added meta tags for the site I'm developing (and in google search it just shows the title of my site with no description), but would it be useful to include meta description tags for each page on my site (and since the site is kind of like a user generated blog, I guess I would need to dynamically populate the meta tag)? Thanks
If Google can find some text on the page that contains the user's search terms, it will display that, and highlight the search terms.
If it can't find such text, it will use the meta description.