What is the difference between Delete and Ripple delete?

Right-clicking on a layer will give you the option to either Delete or perform a Ripple delete

Deleting an element will simply delete it, leaving the space it was occupying on the scene timeline. 

Ripple deleting will delete the element and also the gap it leaves behind on the timeline. 

Note: The Ripple delete option will only work if there is no other element occupying the same space on the scene timeline.