Anaheim peppers are a mild variety of chile pepper used in Mexican and Southwestern cooking.
Canned green chiles, mainstays of American-Mexican cuisine, are made from Anaheim peppers. Indeed, Anaheims are named after the city in Southern California where they were first grown commercially by a businessman named Emilio Ortega, who founded the company that still sells canned green chiles under his name.
Anaheim peppers actually originated in New Mexico, where a slightly hotter version is cultivated to this day — though they go by the name of "New Mexico chiles," or sometimes "Hatch chiles," Hatch being the name of a town in New Mexico known for its chile crop. They say history is written by the winners, though, so we have the Anaheim pepper.
Speaking of history, you might think of Anaheim (if you think of it at all) as the home of Disneyland, maybe a couple of sports teams, not to mention its impressive convention center.
At one time, up until the 1950s, Anaheim was wall-to-wall orange groves. The sunny climate and warm temperatures that made Anaheim ideal for growing oranges also happened to be the ideal conditions for growing chile peppers. Oddly enough, the Ortega operation got its start in Ventura, CA, which is nowhere near Anaheim.
Peppers are a fruit, and like any fruit, they come in a wide array of varietals and hybrids. Even within the category known as Anaheim, varietals with names such as "Big Jim" and "No. 9" are typical.
Anaheims can be harvested while unripe (i.e. still green), which is how the bulk of them are used in the canning operation. After roasting, peeling, and seeding, the chiles are canned, and they are remarkably stable.
They can also be left to ripen on the plant until they turn red, in which case they're referred to as chile colorado (the word colorado means "red" in Spanish) or California red chiles. These can be dried and ground for use as a spice, or as a component of spice blends like chili powder.
Anaheim peppers register between 500 and 2,500 Scoville heat units on the Scoville Scale, which is certainly mild enough to eat raw. The variation in heat is mostly related to differences in soil and the amount of the sunshine the plants get. More sun equals a hotter chile.
Like Poblano peppers, which they resemble, Anaheim peppers are often used for making one of the most popular American-Mexican specialties, the classic chile Relleno, where the pepper is roasted, stuffed with cheese, then coated in egg and fried. This is mainly because they're big enough to stuff and because they have a roughly equivalent heat level. Anaheims are rather narrower than Poblanos, with a lighter green color and a fruitier flavor.