What pepper is used in Sriracha sauce?
Huy Fong Sriracha Sauce is made with a specific variety of red jalapeños which, at this point, is grown only for them.  

Note that this is different from traditional Thai sriracha, which is made fruitier and less hot peppers like the goat pepper.  If you haven't had this, I recommend picking up a bottle to keep alongside your Huy Fong, it's quite different tasting.

Non-traditional sriracha sauces, now made all over the world, may be made of any red, fleshy, hot pepper you can imagine. There's even a few green ones.