Essex is straight veg tan, using the same veg tan liquors used by Horween in tanning shell cordovan. Although it's straight veg, it's not a firm tannage for carving like Wickett and Craig or Hermann Oak. It's more of a medium temper.
Dublin is Essex that has been heavily fat-liquored with fats, waxes, and oils to give it a nice pull-up effect.
Derby is Dublin that has been tumbled in a drum to give a more rustic look and a softer temper.
The entire Essex line is full grain non corrected leather.
Chromexcel is combination tanned. Chrome is the primary tannage followed by a heavy vegetable retannage. That gives it some properties of both tannages. Heat resistance of chrome, ability to hold stamping similar to straight veg, and the edges will burnish nicely. It is hot stuffed with waxes, greases, and oils to give it a slick, classic look, and a firmer temper than fat-liquored leather. It is also a corrected grain leather, so the character is not quite as pronounced as the Essex line.
On a side note, in my personal opinion, the Essex line has more of a classic leather smell that is really nice.