Twin-tube Low Pressure Gas
A shock absorber or strut can be a low pressure gas, twin tube design. This basic design has not changed too much since the 60's, but there have been some refinements over the years. This design has some characteristics, such as a soft valving, and usually costs less to manufacture than a Monotube or Adjustable type shock absorber. They are usually pressurized with some Nitrogen gas, usually 100 psi or less. In this design the hydraulic oil and gas are mixed together in the same chamber, which, while dampening, causes some aeration/foaming. Monroe Auto Equipment, Bilstein, and KYB GR2 still make units like these.

Mono-tube High Pressure Gas
A shock absorber or strut can be a high pressure, mono tube design. These are a more modern type design, and have characteristics of their own. The monotube design allows them to operate cooler. Monotubes are under high pressure, from around 200 psi, to as high as 360 psi. The hydraulic oil and Nitrogen gas are in separate chambers, separated by a floating piston. This allows the shock/strut to function without any aeration or foaming. Monotube shocks usually have a stiff valving, and traditionally cost more to manufacture. They are also able to withstand more punishment, and offer higher dampening ability. (for instance, most racing shocks, from Nascar to Formula One, are of a monotube design). Great care and engineering is taken into consideration when manufacturing a monotube, and many have a very specific valving. This is years of racing technology adapted for street and off-road use. Some of the companies that make monotube shocks are Bilstein , Edelbrock, and KYB.