String is a final class which represents a character
string,string are constant they value cannot be changed
after they r created
its is final class Stringbuffer,Stringbuffer implemets
muttable String character,its is like a string but can be
modified, the length and content can be changed during
method call

What Ravi said is correct in string and string buffer. 
method overriding is same method available in both base or 
super class and in drived class.  At that time we say this 
is over riding. method over loading is same method with 
different signurate and different parameter.

Method over riding ===base class and drived class
Method overloading === same class

When you create a string object, you are creating a string
that cannot be changed. this once a string object has been
created, you cannot change the character that comprise that
string. at first that may seem to be a serious restriction.
However, such is not the case. You can still perform all
types of string operations. The difference is that each time
you need an altered version of existing string,a new string
object is create that contains the modifications. The
original string is left unchanged. this approach is used
because fixed, immutable strings can be implemented more
efficiently than changeable ones. For those cases in which a
modification string is desired, Java provides two options:
StringBuffer and StringBuilder Both hold strings that can be
modified after they are created