The jerkin was an every class garment. It was more vest-like and sometimes, for that reason, worn on the outside of the other garments, or just by itself, as well.
The jerkin can be ornate like the doublet too, however, jerkins didn’t have sleeve which is why they were more vest like and could be worn over the doublet. That is the big difference. No sleeves! For lower classes the jerkin was worn by itself over the tunic. (A note about a good character who wears a good example of a jerkin is Aragorn from Lord of the Rings.)