There are literally thousands of masks on the market. Endless options for all skin types and all skin conditions. So where to begin"

Masks were traditionally a quick fix or an extra boost, normally used once a week to help possibly fix whatever issue you had.
These days they're much more sophisticated and can be left on overnight, used as a 15 minute quick fix, or used under moisturisers during the day if you so desire.

So what type of mask works best for which skin" More in-depth reviews will follow but for now the briefest of overviews:

Clay
The original and most well-known type of mask, clay is traditionally used for deep cleaning, decongesting, in general it's the one you grab when you feel a bit dirty, like you need a really good clean. Best for:
oily, combination, blackheads, breakouts, dull
Avoid if:
Check the ingredients for magnesium and avoid if you're sensitive - it can be very stimulating.

Exfoliating
These are either bead-based (I don't recommend microbead masks, I'm talking about silica/jojoba etc), enzyme or acid-based and are used to resurface the skin, slough off dead skin cells, turnover the stratum corneum (15-20 layers of dead, flattened, skin cells) a little quicker to give you smooth skin.
Best for:
dull, flaky, sallow, dry, dehydrated
Avoid if:
sensitive, easily reactive

Moisturising
Moisturising masks are usually used last and add oil-based moisture to the skin. They're the perfect mask to sleep in if you're older and contain thing...