Flickr introduced a new feature: Collections. It is sorta a math/programming term, but basically you now can organize your Sets into Collections. Collections can either contain other Collections OR Sets, not a mix of both, this I think is kindda strange.
To clarify the concept of Flickr Collections, basically a Set in Flickr is a set of your images, and a single image can be in multiple sets. In most other photo hosting sites or site software, they usually have the concept of albums instead, but in most cases a given image can only be in one album unless you manually add it to more than one. Personally, I like the concept of Sets more. But it becomes anonying when you have too many sets, so the concept of albums and sub-albums becomes desirable. Naturally, they came up with Collections, which are more general than sets, that Collections can contain other Collections or Sets (but not both!). So this gives you something similar to albums and sub-albums but it is more general, because a collection can obviously be in multiple collections. Well, I think this might be confusing to some still, but most of my readers have some kindda math, engineering, or computer science background.
More on Sets and Collections at Wikipedia.
They also allow you to customize your Flickr page layout, you can choose to display sets or collections, or just images. And you can also choose to display the images in medium or small sizes.
What are collections, you ask? A collection is a container into which you can place either sets or other collections, allowing you to create a hierarchy as deep as 5 collections. You can place as many of your sets into a collection as you like, and a set can be in as many different collections as you like.
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