Both String and String Builder are classes used to handle strings.
string is concatenation can performed very efficiently.when concatenation is done it creats 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 "String Builder" object, the Append method is used. This means, an insertion is done on the existing string. Operation on String Builder object is faster than String operations, as it is done at same location. Usage of String Builder is more efficient in case large amounts of string manipulations.