The document defines various string functions in C# including Clone(), CompareTo(), Contains(), EndsWith(), Equals(), GetHashCode(), GetType(), GetTypeCode(), IndexOf(), ToLower(), ToUpper(), Insert(), IsNormalized(), LastIndexOf(), Length, Remove(), Replace(), Split(), StartsWith(), Substring(), ToCharArray(), and Trim(). It then provides an example program that demonstrates how to use these functions by taking user input strings and displaying the results of applying each function selected from a dropdown menu.