I've found a couple of Marks so far in Arena, but looking around the UESP it doesn't really list what they are. In their listing there it lists spell effects for each mark, as well as price and where they can be bought, so are they like scrolls? How do you use them?

Equip then press "u" to use them, the number of charges list how many uses are left. Identify at the Mage's Guide to learn the effect. Crystals, rings and bracers work the same way.

If you are asking as to what a mark physically is, a clue comes from Daggerfall, where they have actually sprites for such items. It looks to me to be like some sort of card, or piece of paper, that is enchanted. I suppose it is fastened to your garments in some way.

Think of it as something like those little Chi strips with chinese calligraphy used in some fictions like the Jackie Chan cartoon series.

Yea, I was thinking a mark was some sort of decorative pin or something. Nobody really knows.

I always thought they were enchanted gemstone things that emitted magical beacons or something...of course this is a result of me mixing up Arena and Daggerfall's marks up with the spell from Morrowind.