A slide element is one or more shapes on a slide, that inserted from the element library into a slide in the active presentation. 

The difference between the slide library and the element library is that a slide library contains  whole slides for insertion, whilst the element library contains just graphics. 

A slide from the slide library:

An element from the element library:

There are different types of elements available:

1. Simple icons or graphics

2. Single graphic elements

3. Graphic elements grouped together