in a few words collections are container classes in which you can put in all kinds of objects. Most collections are optimized in a special way regarding the performance of some operations (insert, delete, search, ...) or the features you get from a particular collection.
Basically there a 3 or 4 base interfaces: Collection, List, Map and Set. For each of these there are maybe more implementations for special purposes ranging from performance to concurrency aspects.
Here's a good online tutorial which explains collections in more detail. Have a look at it, I think it's quite better than I could explain here in a few lines: