string is immutable, that is it can not be extended.whereas
StringBuffer is mutable and can be extended.
Fro example :
  Consider 2 statement : "Welcome "  and "to Java World".
 now assign first statement to string and string buffer.
  String str = "Welcome" & StringBuffer strBuff = new
StringBuffer("Welcome");
  Now if we add 2nd statement to both then :
  str= str + "to Java World" -> In this case, it would dump
all the memory allocated with "welcome" and allocate a new
memory space to the entire string "Welcome to Java World"  .
On the other hand, in strBuff  :-
  strBuff.append("to Java World") -> if simply allocate a
new memory space to only 2nd statement and add to link to
previous name.

String is immutable that once the value assigned to object 
cannot be modified.Suppose if we want to perform 
concatenation another string object will be prepared and 
the value is going to assign for that object.in total we 
are having 2 objects

StringBuffer is Mutable we can change the value of the 
object even the value is assigned to the object.

Simple Example for String And String Buffer.

String must allow the declared memory space to the variable.

String Buffer automatically increase the Exists memory space

String is Immutable and String buffer is mutable.
Inside String.java, equals() method is overrideen but not 
the case in StringBuffer.

This all is the simple.