String是固定不变的，而StringBuffer和StringBuilder可以改变它们的值。

StringBuffer和StringBuilder唯一的区别是StringBuilder是非同步的而StringBuffer是同步的。当系统需要运行单线程时，使用StringBuilder比较好。StringBuilder比StringBuffer更有效用。

选择原则：

1. 如果你的文本（字符串）不会改变就用String类因为String对象是不可变的。
2. 如果你的文本（字符串）可以改变并且在单线程中访问，使用StringBuilder。
3. 如果你的文本（字符串）可以改变并且在多线程中访问，使用StringBuffer。

 

原文：http://www.java-tips.org/java-se-tips/java.lang/difference-between-string-stringbuffer-and-stringbu.html

String is immutable whereas StringBuffer and StringBuilder can change their values.

The only difference between StringBuffer and StringBuilder is that StringBuilder is unsynchronized whereas StringBuffer is synchronized. So when the application needs to be run only in a single thread then it is better to use StringBuilder. StringBuilder is more efficient than StringBuffer.

Criteria to choose among String, StringBuffer and StringBuilder

1. If your text is not going to change use a string Class because a String object is immutable.
2. If your text can change and will only be accessed from a single thread, use a StringBuilder because StringBuilder is unsynchronized.
3. If your text can changes, and will be accessed from multiple threads, use a StringBuffer because StringBuffer is synchronous.