In order to ensure a proper air mixture in an engine, valves are utilized at the top of each cylinder. Controlling those valves is a camshaft. The camshaft spins when the engine is running to control valves that let air into the engine to aid in combustion and let out subsequent exhaust gases (via CarParts.com).
With a few exceptions, new engines generally come in two flavors: engines that utilize a Single-Overhead Camshaft (SOHC) and those that use a Dual-Overhead Camshaft (DOHC). SOHC engines are generally more simply laid out and can run more efficiently than some other DOHC engines. Honda still uses SOHC engines to great effect in their lineup. But SOHC engines are often not the best choice for high-horsepower applications as there can only be two or three valves per cylinder.
On the other hand, DOHC engines often make more horsepower out of smaller displacement engines than SOHC configurations. DOHC configurations allow for up to four valves per cylinder. Sadly, that extra power can come at the expense of complexity, adding even more parts that can fail. All the same, most manufacturers have gone with a DOHC setup for the vast majority of their engines.