The Dichotomy of Mutable and Immutable in Java

String and StringBuffer are immutable and mutable analogues of one another. StringBuffers are used to implement the binary string concatenation operator.

Example: x = "a" + "b" + "c";
is compiled to x = new StringBuffer().append("a").append("b").append("c").toString();

You can also call append on ints, chars, floats and doubles.

When you instantiate a StringBuffer it has an initial capacity of 16 characters, however e.g. new StringBuffer(500) will create a StringBuffer with an initial capacity of 500 characters.

An interesting article on a mutable and immutable design pattern (as exemplified by String and StringBuffer) can be found on javaworld.