There are currently two (2) types of designs for modern four cycle internal combustions engines:
1. OHV - This stands for "Overhead Valve", meaning the intake and exhaust valves are located in the cylinder head(s) and above the cylinder(s) and combustion chamber(s). The valves are operated by a camshaft that is located in the "V" portion of the cylinder block, between the intake manifold and the crankshaft. This makes for a long distance between the camshaft and the valves. The camshaft is connected to the valves by a lifter, (either solid or hydraulic) then a pushrod and finally a rocker arm. (Thus the term "pushrod" engine.) As the camshaft revovles the high portion(s) or lobe(s) of the camshaft come into contact with the lifter(s) pushing upwards, which then moves the pushrod(s) up, which in turn pivots the rocker arm(s) into contact with the valve stem(s) causing the appropriate valve(s) to open.
2. OHC - This stands for "Overhead Cam", meaning the camshaft(s) are located in the cylindr head(s), not in the cylinder block. (NOTE: OHC engine designs are also OHV designs because the valves are located in the head as in an OHV. However, OHC engine designs are never termed OHV.) In an OHC design the pushrod(s) is/are eliminated because the camshaft(s) set(s) directly above the valve(s) in the head. Usually there is no lifter(s) in an OHC design, though, I think I recall an OHC design in the past with lifters, but cannot recal the manufacturer. The OHC design, by eliminating the complicated OHV valve actuating method, has the advantage of being able to rev up to a higher RPM power producing limit, than an OHVV design, through weight savings, friction reduction and the ability to reduce valve float. There is less strain placed on the valve train. As a result, the OHC design is usually a smoother operating engine. They are more expensive to manufacture mainly due to the additional casting required in the cylinder heads to hold the camshaft(s), multi-camshafts and the drivetrain to operate the camshaft(s). OHC engines are made SOHC (Single Overhead CAM) and DOHC (Dual Overhead Cam) designs.
Prior to the invention and popularization of the OHV and OHC engine designs there was the flathead or "L" head design. This had the valves mounted in the cylinder block, setting upside down off to one side of the cylinder. The camshaft was in the block directly below the valve stems. This was a very inefficient and power restrictive design.
NOTE: A camshaft NEVER comes into direct contact with the valve stem in any engine design. There will always be an adjusting shim, adjustor mechanism, a rocker arm or a cap between the lobe of the camshaft and the valve stem. So, while a camshaft pushes against the valves, it does so indirectly, though the OHC design is as close to direct contact to the valve as is possible.
Pushrods are of two (2) designs. The first is a one piece hollow tube and the second is a three (3) piece hollow tube. In the first design the pushrod is simply a mechanical connection between the lifter and the rocker arm. The valves are adjusted by a screw and locking nut located in the rocker arm or if hydraulic llifters are incorporated, the valves are adjusted automatically by the lifter. In the second design, the pushrod is not only the mechanical connection between lifter and rocker arm, but also the mechanical adjusting mechanism for the valves. This design incorporates a hollow pushrod that is female threaded to accept a short threaded shaft that has a locking nut on it. The valves are adjusted by loosening the locking nut and turning the the short shaft that is threaded into the top of the pushrod. NOTE: Some manufacturers, like Ford, use diferent length pushrods to adjust the valves.
A bit long winded, but I hope this helps you to better understand the differences between an in engine designs.