string is imutable and stringbuffer is mutable
 
Mutable means storing location is same
Imutable means storning location is different

hi
 i am agree with Madhu , 
String is an immutable object. StringBuffer is a mutable 
object. 
immutable object:If you are not allowed to change the value 
of an object, it is immutable object.

mutable object: If a object value is changeable then we can 
call it as Mutable object.

String and StringBuilder class stores strings. But when you
cannot change a String object after creating one.
eg: String name = "Prasad";
By saying you cannot change the name object means you cannot
change the value in name object internally. When you change
the name object value to something else, a new String object
is creating in memory and assign the new value.

eg: name = "Prasad Reddy";

A new name object is creating in memory and the value
"Prasad Reddy" is assinging to the newly created space.

But StringBuilder class occupies the same space even if you
change the value.

If you are doing string concatenation StringBuilder class is
far better in performance than String class.

You can use StringBuilder's Append() method to use
concatenation.