Similar to photo albums, collections are a useful way to group images together. You can make arbitrary collections of images, or you can make collections based on search results (Smart Collections).
To manage collections of images, click on the Collections panel (if it isn't visible, select it from the Window menu) and do one of the following:
To create a new collection: Click the plus button at the bottom of the Collections panel and give your collection a name.
To add photos to a collection: Drag images from the Content area to your collection's name in the Collections panel.
To remove a photo from a collection: Select the photo and click the Remove From Collection button that appears at the top of the Content area.
To delete a collection: Highlight the name of the collection and click the trashcan icon on the lower right of the panel. Note that when you delete a collection you're not actually deleting the photos it contains from your hard drive, you're just deleting that specific grouping of photos.
Smart Collections are like regular collections, except they're ...well, smart. You can build a collection based on criteria that you specify, like ISO (film speed), focal length, or keywords, and have Bridge search for images (in a specific folder, or even your entire computer) that meet those criteria. The "smart" part is that Smart Collections update dynamically, meaning that as you add or remove images ...