A string is a sequential collection of characters that is used to represent text.
A String object is a sequential collection of System.Char objects that represent a string; a System.Char object corresponds to a UTF-16 code unit.
The value of the String object is the content of the sequential collection of System.Char objects, and that value is immutable (that is, it is read-only).
For more information about the immutability of strings, see the Immutability and the StringBuilder class section in msdn.
The maximum size of a String object in memory is 2GB, or about 1 billion characters.
Note : answer is extracted from msdn help section. You can see the full content here in msdn String Class topic under Remarks section