Difference between String and Stringbuilder in Asp.net,C#

String
 
String is immutable, Immutable means if you create string object then you     cannot modify it and It always create new object of string type in memory.
 
Example
 
 string strValue = "Hello Friend";
 // create a new string instance instead of changing the old one
 strValue += "Are You ";
 strValue += "Ready ?";
 
Stringbuilder
 
StringBuilder is mutable, means if create string builder object then you can perform any operation like insert, replace or append without creating new instance for every time.it will update string at one place in memory does not create new space in memory.

Example
 
 StringBuilder sbValue = new StringBuilder("");
 sbValue.Append("Hello Friend");
 sbValue.Append("Are You Ready ?");
 string strValue = sbValue.ToString();
 

String	StringBuilder
immutable	mutable
Performance wise string is slow because every time it will create new instance	Performance wise stringbuilder is high because it will use same instance of object to perform any action
don’t have append keywor	can use append keyword
belongs to System namespace	belongs to System.Text namespace