Matt Cutts Discusses Snippets 2012 Tutorial of Internet’s Search
The snippet on the search engine results page appears below the Page Title.
Google tries to find the most relevant part of the page.
Sometimes it is a single snippet.
Sometimes it comes from multiple places.
In many cases, the Meta description tag is chosen.
It is worthwhile AB testing to see if greater click through rate can be achieved.
Words in the Page Title and Snippet that match the search keywords are emboldened.