mmmmm, yesss.....well, they are kind of different. The peel eats away the top layer of the skin (chemical exfoliation via salicylic, glycol, or fruit acid and may contain little particles for physical exfoliation) to reveal the fresher/brighter/smoother/dewy-er skin underneath. The mask either pulls stuff out of the skin (clay/charcoal/mud masks for acne/pores etc) or deposits fancy goody stuff into the skin (hydrating/brightening/anti-aging masks for soothing/hydration/anti-aging).
And then there's the new guy on the block called peel masks, where you apply a layer, it dries into a super thin rubbery sheet, and you peel it off (Boscia black and white masks, Sekkisei Kose, True Blue Spa from BBW). They are usually meant to get rid of flaky skin and make it feel smooth without drying it out.
Just like Beautytester states, a peel is better at removing/taking off things while a mask can do either removing/taking off or infusing/locking in.
Peels also generally will have a more gel-like feel while masks range from gel formulas to thicker creamy/mud formulas.