Google crawlers read your web page and parse the title and description tags. If the description matches the keywords intended for this page, then it uses the description tag for information on the search page. Otherwise, Google crawlers try to see the first H1 tag to be the title, it also look for next p tag to be the listing description. Sometimes, it concatenates H2 tag w/the p tag to make up the description (up to 168 bytes).
Also note that Google does all sorts of things to your title. It might completely rewrite the page title if it feels like it, and if it thinks it can do a better job. Google sometimes replaces the title with your URL or parts of your URL, but it’s also known to append the domain name to the end of your title.