In C#, the string is an object of the System.String class that represents a sequence of characters. Strings support many common operations like concatenation, comparison, getting substrings, searching, trimming, and replacing. The string keyword is an alias for the System.String class, so string and String can be used interchangeably. Methods like Compare, Contains, IndexOf, Split, Trim, and ToUpper allow manipulating and examining the value and characters of a string.