mutable string and immutable string

In C#, all strings are immutable - which is a fancy way of saying "can't be changed" (mutable means it can be changed).
It may look as if you are changing a string but every time you perform an operation on a immutable object, it happens to a copy of the the string and a new string is returned.
For example:

string s1 = "hello ";
string s2 = s1 + "there ";
string s3 = s2 + "Joe!";
Console.WriteLine(s1);
Console.WriteLine(s2);
Console.WriteLine(s3);

Because strings are immutable, creating s2 does not affect s1, and similarly creating s3 does not affect s2 or s1:

hello 
hello there 
hello there Joe!

Once you create a member of the string class, it will never be changed. A StringBuilder on the other hand is mutable - you can add and remove data from it, and it is only ever copied when the space allocated is too small to hold the new data.