Tacos al Pastor, much like the Greek Gyro or the Turkish Döner Kebap, are carved off of large cones of layered meat (usually pork). The yellow bit at the top is pineapple (a chunk gets cut off to garnish each taco), and the bottom is help up by large onions.
El Tizoncito claims to have invented Al Pastor. While they certainly have been around for a while, these days they have a hard time claiming much street cred - what with logo-emblazoned staff and a flash-heavy website.
Still, their tacos are pretty good. We had their "Pastor Al Centro", a combo plate with cebollitas (grilled small onions), chilangos (corn tortillas topped with frijoles and pastor), pollitas (flour tortillas with cheese and pollo al pastor) and tacos de pastor.
They also give you a very attractive and tasty assortment of salsas, along with chips and a nice warm bean sauce.
The taco de pastor in all its glory, garnished with onion and cilantro.
Here is a short video of the tacos being cut: