String	String Buffer
String is a data type and class in java	String Buffer is a class in java
String is Immutable	String Buffer is Mutuable
String comes under character datatype	String Buffer comes under reference data type
In concatenation operation String is slower	In concatenation operation String Buffer is faster

Difference between StringBuilder and StringBuffer in Java

StringBuffer is very good with mutable String but it has one disadvantage all its public methods are synchronized which makes it thread-safe but same time slow. In JDK 5 they provided similar class called StringBuilder in Java which is a copy of StringBuffer but without synchronization. Try to use StringBuilder whenever possible it performs better in most of cases than StringBuffer class. You can also use "+" for concatenating two string because "+" operation is internal implemented using either StringBuffer or StringBuilder in Java. If you see StringBuilder vs StringBuffer you will find that they are exactly similar and all API methods applicable to StringBuffer are also applicable to StringBuilder in Java. On the other hand String vs StringBuffer is completely different and there API is also completely different

Performance Measures between String , StringBuffer, StringBuilder

public class builder {
    public static void main(String[] args) {
        int N = 100000;
        long t;

        {
        	 String sb1 = new String("hello");
             t = System.currentTimeMillis();
             for (int i = N; i --> 0 ;) {
                 sb1.concat("hi");
             }
             System.out.println("time take by String :" +(System.currentTimeMillis() - t));
        	
        	
        	
            StringBuffer sb = new StringBuffer("hello");
            t = System.currentTimeMillis();
            for (int i = N; i --> 0 ;) {
                sb.append("hi");
            }
            System.out.println("time take by StringBuffer :" +(System.currentTimeMillis() - t));
        }

        {
            StringBuilder sb = new StringBuilder("hello");
            t = System.currentTimeMillis();
            for (int i = N; i --> 0 ;) {
                sb.append("hi");
            }
            System.out.println("time take by StringBuilder :" +(System.currentTimeMillis() - t));
        }
    }
}

When we concate a string "hi" in String object, StringBuffer Object and StringBuilder object then time taken is: