Just to go with the formality flow, let's first define a snippet. According to wikipedia, a Code Snippet is a programming term for a small region of re-usable source code or text. Ordinarily, these are formally-defined operative units to incorporate into larger programming modules. Snippets are often used to clarify the meaning of an otherwise "cluttered" function, or to minimize the use of repeated code that is common to other functions.
Visual studio allows us to add our own code snippets for reuse of common block codes in current or further projects. Custom code snippets can be adhered to the ones bundled with the IDE. These can be created and edited with the text editor of your preference. Snippets are handled as XML files, which follow a XML Schema from Microsoft. This schema contains the definition of mandatory elements and attributes for the snippet to work.