String bulider Can Be Used When More Than One String Can we 
concatenated.
StringBuilder which is more efficient because it does 
contain a mutable string buffer. .NET Strings are immutable 
which is the reason why a new string object is created 
every time we alter it (insert, append, remove, etc.). 

StringBulider Is more Effiecent Then String B'Cuse It 
Provide Some Standered Function like Append,Reverse,Remove
etc.

Strings are immutable means Data value maynot be Changed and
Variable value may be changed.
StringBuilder performs is faster than Strings. and also
designed for Mutable Strings
we can use like  this
System.text.StringBuilder

String are Immutable (Not Modifiable). If you try to modify 
the string it actually creates a new string and the old 
string will be then ready for garbage collection.

StringBuilder when instantiated, creates a new string with 
predefined capacity and upto that capacity it can 
accodomate string without needing to create a new memory 
location for the string....i mean it is modifiable and can 
also grow as and when needed. 

When the string needs to be modified frequently, preferably 
use StringBuilder as its optimized for such situations.

Both String and StringBuilder are classes used to
handle the strings.

     The most common operation with a string is
concatenation. This activity has to be performed very
efficiently. When we use the "String" object to concatenate
two strings, the first string is combined to the other
string by creating a new copy in the memory as a string
object, and then the old string is deleted. This process is
a little long. Hence we say "Strings are immutable".

     When we make use of the "StringBuilder" object, the
Append method is used. This means, an insertion is done on
the existing string. Operation on StringBuilder object is
faster than String operations, as the copy is done to the
same location. Usage of StringBuilder is more efficient in
case large amounts of string manipulations have to be performed.

String class is immutable, means we can not change the
contents of string at run time,
for example
String s1="loin";
string s2=s1.insert(3,"g");
now s2 have the value login ,but one thing to notice here is
that we are not assigning this value into s1, because it is
not possible in String class
,but if we want to change the contents of s1 then we will
have to take the StringBuilder calss 
because with the help of this class we can change the
contents of same string
examlpe:
String s1="loin";
s1=s1.insert(3,"g");
now s1 is "login"
here we are assigning the value again in s1,
this is the main difference between in string and
stringBuilder class