XML comments are a structured way to document the types and members within your code. If you use this, the compiler will generate XML documentation for your types and members, which can then be processed via a tool like Sandcastle to build help automatically. The information in the generated XML files is also used by Visual Studio (to provide intellisense).
You can use multiline delimiters via:
/** <summary>
Your info here
*/
Although the tooling doesn't generate it in this form.
For details, see XML Documentation Comments on MSDN.