The denim kind are usually sleeveless. The leather kind are usually a converted biker jacket.
It is covered with sew-on patches with the names/logos of favorite bands. There is usually also a large backpatch covering the back of the jacket, with almost all the available space left taken up by smaller patches. Studs and/or spikes are optional, but they aren't punk jackets so don't go overboard.
Some may smell a bit due to being worn exclusively in moshpits where it can get sweaty at times.
Metalhead 2: Yeah I know. I dare you to smell it though!