Are endothelium cells essentially, epithelium cells

Please explain the difference, I don't understand.

I agree, bloody confusing stuff. Very vague specifications on the internet....

Anyway.

Epithelium cells line the cavities and surfaces of things in the body.

Endothelium cells line specifically the lumen of blood vessels and lymphatic system.

i.e. endothelium cells are a type of epithelium cell.