Slideshows are much more than just a random collection of pictures. In fact they are much more than a connected collection of pictures.
By definition they are a group of pictures viewed in a sequence. They can however be used to convey messages, advertise products, illustrate ideas and demonstrate artistic content among other things.
They can be as simple as a WordPress slideshow:
They can have Musical backgrounds:
They can even be complete elaborate productions advertising a product or an event like this commercial for the Journalism Education Association National Convention produced by Brian Snyder of the Walter Cronkite School of Journalism and Mass Communication.