The IOE (Intake-Over-Exhaust) or, in American parlance, the "pocket-valve." Because this was the way Count de Dion's mechanic, Georges Bouton, laid out their first and very widely-copied engine, many engine-makers adopted it worldwide, among them Indian, Harley, and Curtiss. The exhaust valve is located beside the cylinder, stem-down, and the intake is directly above it, stem-up, in a little cage. In its original form, the intake was suction-operated against a light spring, but later a cute little rocker was perched on one edge of the cage, also operated by pushrod.
The T-head, which placed the exhaust valve on one side of the cylinder bore, stem down, and the intake valve on the opposite side of the cylinder, stem down. This gave the cylinder head an Alfred E. Neumann shape–a big circle with a round "ear" on either side.
The L-head, aka "flathead," or in British lingo, "side-valve." This placed intake and exhaust valves on one side of the cylinder with their stems down, giving the combustion chamber the plan form of three circles blended together (one for the cylinder bore and two smaller ones next to it for the two valves).
OHV, for Over Head Valves, which are located not in the cylinder casting as in the above three types, but in the cylinder head, with their stems pointing generally upward.