Vermeil (said ver-may) jewelry has a thick layer of gold over solid sterling silver. Here in the US, that layer of gold must be at least 2.5microns thick (in other countries it is less, so you might want to be cautious of where you're purchasing your vermeil jewelry). Here at Verdia, we use 23.5K gold. Some vermeil can include a nickel layer between the sterling and gold layers, but all Verdia Jewelry designs are nickel free, as we want to be as hypoallergenic as possible.
The gold layer on Vermeil is plated over the silver. Plated is one of those panic-inducing words because plated jewelry includes some really terrible quality pieces - the kind of stuff that flakes, causes allergic reactions, and turns your skin green. Vermeil will not do any of that because the layer of gold is vastly thicker than most plated jewelry, which gets just the thinnest layer of gold, called flash plating.
Vermeil is a little different from gold filled, which is another of my favorite hybrids. Gold filled jewelry is usually 5% 14K gold by weight. For heavy pieces, this translates to a nice, thick layer of 14K, but for thinner pieces, 5% can actually contain less gold than a plated option like Vermeil. Once you add in that the core in gold fill is brass, and that the gold layer is 14K rather than 23.5K, thin gold filled pieces are unlikely to have the same precious metals value and longevity that Vermeil does!