Definition: Snippet
Name given by Google to the text that appears below a web title in the search results. It may come from the meta description, from Dmoz or from the page content itself.
Definition: Sitemap
As the name indicates it is a map of the site, it contains a list of URLs that need to be indexed and it also contains other information for the robots.
Definition: SERP (Search Engine Results Page)
As the name indicates it is the page where search engine results appear.