Be sure to read the source code for this example for additional explanation of the terminology.
Containers vs Empty Elements
Container Elements have both a start tag and an end tag.
For example, once you start bold, you have to stop it with a end tag or the bold will never stop.
The formatting induced by the element is applied ONLY to the contents of the container.
Empty Elements only have a start tag -- no end tag.
The only empty elements formally introduced in this lesson are line breaks (br) and horizontal rules (hr).
They are basically self-contained instructions (just do it) that don't require a closing tag.
Nesting of Container Elements
Nesting means putting one container inside another container - like putting one box inside another.
See in the source code how this is both bold and italic and also so is this (reverse nesting order)
and for more info about nesting.