My understanding is like Emily's: essentials are workhorses, but not all workhorses are essentials. For me, essentials are frequently worn but are not statement pieces. They are items that go with lots of other stuff in my closet. For instance, if I buy a new skirt, I know that if I can't find a special top to go with it, one of my black or white tees will do the trick. And likewise, if I buy an amazing top, I know that I can wear it with one of my pairs of black pants or one of my pairs of blue or white jeans. The tees and the jeans are essentials for me, because they allow me to easily wear all of my wardrobe.
I often use essentials to set off statement pieces as well, since I want the statement piece to be the center of attention. Essentials are usually pretty basic and so are good for this.