1) Epoxy Primer is a two part primer consisting of the primer and an activator.
2) Epoxy primer is used as a direct to metal primer (DTM) because it has excellent corrosion resistant properties.
3)Depending on the brand it also can be reduced and used as a paint sealer. Also depending on the brand used there are several colors available such as black, white, gray, green and red.
4)Epoxy primer works great over freshly sand blasted metal. It is not meant to spray over rust, the metal must be properly sanded or blasted and clean.
5)Epoxy primer will adhere to all properly prepared metals. I have used it on steel, stainless steel, aluminum and brass.
6)It can be used underneath body filler.

I have personally used PPG, Lesonal and U-Tech. The Lesonal and U-Tech have worked best for me as opposed to the PPG because they are able to be sanded without clogging the sandpaper. They also have some filling properties so they work great on small parts that you only want to prime once without having to spray again with a filler primer.

I have gathered some tech sheets on a few various brands for further reading.

Also visit this page for more information.
Crankshaft Coalition/Epoxy Primer

Will will need Adobe Acrobat view most of them.

Dupont Chroma System Epoxy
Dupont Nason Epoxy Primer
Dupont Hot Hues Epoxy
Lesonal Epoxy
Sikkens Autocoat LV
Sikkens Auto Surfacer EP
Akzo Nobel U-Tech E380
Akzo Nobel U-Tech E350
PPG
PPG Omni Line
Southern Polyurethanes
Matrix Systems
Martin Senour
Kustom Shop DTM
House of Kolor Chromate Free