OHV, OHC, SOHC and DOHC (twin cam) engine design

4-cylinder inline 8-valve OHV engine

OHV means OverHead Valve - an engine design where the camshaft is installed inside the engine block and valves are operated through lifters, pushrods and rocker arms (an OHV engine also known as a "Pushrod" engine). Although an OHV design is a bit outdated, it has been successfully used for decades. An OHV engine is very simple, it has more compact size and is proven to be durable.
On the downside, it's difficult to precisely control the valve timing at high rpm due to higher inertia caused by larger amount of valve train components (lifter-pushrod-rocker arm). Also, it's very difficult to install more than 2 valves per cylinder, or implement some of the latest technologies such as Variable Valve Timing - something that could be easily done in a DOHC engine.

4-cylinder 8-valve SOHC engine

OHC in general means OverHead Cam while SOHC means Single OverHead Cam.
In a SOHC engine the camshaft is installed in the cylinder head and valves are operated either by the rocker arms or directly through the lifters (as in the picture).
The advantage is that valves are operated almost directly by the camshaft, which makes it easy to achieve the perfect timing at high rpm. It's also possible to install three or four valves per cylinder
The disadvantage is that an OHC engine requires a timing belt or chain with related components, which is more complex and more expensive design.

DOHC means Double OverHead Cam, or sometimes it could be called "Twin Cam." A DOHC setup is used in most of newer cars. Since it's possible to install multiple valves per cylinder and place intake valves on the opposite side from exhaust vales, a DOHC engine can "breathe" better, meaning that it can produce more horsepower with smaller engine volume. Compare: The 3.5-liter V6 DOHC engine of 2003 Nissan Pathfinder has 240 hp, similar to 245 hp of the 5.9-liter V8 OHV engine of 2003 Dodge Durango.
Pros: High efficiency, possible to install multiple valves per cylinder and adopt variable timing.
Cons: More complex and more expensive design.