What is the Google Collections Library?

What is the Google Collections Library?: "'Of course, this is a really trivial example. What's great about something like Multimap is that the minute you realize you need something slightly more complex, it's already ready to handle that for you. For example, if you're removing entries as well as adding them, in the first code example you'd have to figure out what to do about pruning those empty collections. With Multimap it just happens.'"