The Collections pane appears in all modules. From the Library, images may be organized into collections — a way of grouping images into virtual collections, without affecting the actual folder or file structure where your images are stored on disk. Collections only exist within your Lightroom catalog. Once you have created a collection, you may then access that group of images from within any of Lightroom’s modules, including the Web module.
The Collections pane may house several types of items:
A Collection is a group of images manually assembled and sorted by the user.
A Smart Collection is a group of images automatically compiled by Lightroom in accordance with a set of rules defined by the user. For example, a smart collection may be setup to include all images from your Library rated with three stars; any image assigned a three star rating would automatically be added to the smart collection.
A Collection Set is a collection of collections, a folder containing several collections. Collection Sets may be used to organize related collections under a single heading.
An Output Creation is similar to a Collection, except that it includes your images as well as your custom settings from one of Lightroom’s output modules — Slideshow, Print or Web — depending upon in which module the Output Creation was made. When made in the Web module, an output create is a “Saved Web Gallery”.
When using Lightroom to create a website, it is a good idea to keep your images organized for web-publishing using a combination of Collection Sets, Collections and Saved Web Galleries.