In the classic version the peppers are blanched, then stuffed with a cheddar, Monterey Jack, or cream cheese mix, then breaded with a milk-egg-breadcrumb mixture and deep fried. They are served with ranch dressing or a berry preserve.
There are extensive variations using ground meat, mushrooms, bacon, or crab, and cooking methods such as baked or grilled. Often they are mass produced and frozen, to be deep fried.
The origin is unclear, but they are most likely an americanized version of the classic Mexican dish, chile rellenos.
Jalapeño poppers are sometimes also known as "Alligator Eggs."