String String is immutable. Immutable means once we create string object we cannot modify. Any operation like insert, replace or append happened to change string simply it will discard the old value and it will create new instance in memory to hold the new value. Example string str = "hi" ; // create a new string instance instead of changing the old one str += "test" ; str += "help" ; String Builder String builder is mutable it means once we create string builder object we can perform any operation like insert, replace or append without creating new instance for every time. Example StringBuilder sb = new StringBuilder ( "" ); sb.Append( "hi" ); sb.Append( "test " ); string str = sb.ToString(); Differences String It’s an immutable Performance wise string is slow because every time it will create new instance In string we don’t have append keyword String belongs to