An album is a collection of photos that you can upload and store online. Many social networking sites like Facebook and Google+ allows you to upload a set of photos and organize them into an album so as to be able to sort your uploads into various events you determine.
Albums can be shared with others and the individual photos can be tagged with your friends in it. An album can also be private together with the photos in it.
Like a physical photo album, such online albums keeps a memory of you and your friends in a particular event that you may have attended years back.