Difference Between String and string

A string is a sequential collection of Unicode characters, typically used to represent text, while a String is a sequential collection of System.Char objects that represents a string. The value of the String is the content of the sequential collection, and the value is immutable.

Each Unicode character in a string is defined by a Unicode scalar value, also called a Unicode code point or the ordinal (numeric) value of the Unicode character. Each code point is encoded using UTF-16 encoding, and the numeric value of each element of the encoding is represented by a Char. The resulting collection of Char objects constitutes the String.

Ref    http://msdn.microsoft.com/en-us/library/system.string(v=vs.71).aspx