what is the difference between string and stringbuilder

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. 

OR

Difference..
String..
1.Its a class used to handle strings.
2.Here concatenation is used to combine two strings.
3.String object is used to concatenate two strings.
4.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
5.we say "Strings are immutable".

String Builder..
1.This is also the class used to handle strings.
2.Here Append method is used.
3.Here, Stringbuilder object is used.
4.Insertion is done on the existing string.
5.Usage of StringBuilder is more efficient in case large amounts of string manipulations have to be performed