A class used to improve the performance of string concatenations. When two strings objects are concatenated, a third object must be allocated to contain the new string. Probably the other two strings will not be used anymore, and stay as garbage on the memory. The way that garbage is removed depends on the browser, but the fact is that some garbage remains. When many concatenations are executed, the performance may be greatly improved by the StringBuffer.
It is internally implemented with an array. Each call to append() generates a new element on the array. When the toString() method is called, the array is joined into a single string.
Because the standard toString() method is used to build the string, when the buffer itself is transformed to a string (like on a alert(buffer), or str = "abc " + buffer), the internal string is used instead of somethig like [Object].

var buffer = new StringBuffer();
buffer.append("This ");
buffer.append("is ");
//Nested callings - the append method returns the buffer itself
buffer.append("a ").append("test");
alert(buffer); //Message: "This is a test"