A snippet is the line of information that appears in a Google search result. A rich snippet is one that is enriched with additional information such as an image, star rating for product reviews, and event details.
Google can display rich snippets if your site provides structured data – data markup that is structured in a specific way so that search engines can interpret it. Schema.org has numerous microformats that help with the creation of rich snippets. They are important for search engine optimization because they have greater visibility in search results than other snippets and tend to have higher click-through rates.
Rich snippets are query-dependent. To be eligible to appear in the results for a specific query, you must implement the correct structured data schema. For example, a page using the event schema won’t be considered if search results display a rich snippet based on the FAQ markup.