Lines starting with '#' are comments. They have the role of describing the code to the developer.
Strings following a line with "def ..." or "class ..." are DocStrings. They have the role of describing the function to the user, as they appear in the interpreter interface, e.g. with help(fun). Strings can be defined with ', ", ''', or """. The latest two are so called multi-line strings. Modules are also documented with DocStrings.