What units are used to measure gas pressure?

The main units are kilopascals (kPa), atmospheres (atm), millimeters of mercury (mmHg), and sometimes Torr.
The kilopascal is a unit for pressure derived from physics. Pressure is defined as a force exerted over an area. The larger the force, the greater the pressure. But, if the force is concentrated over a small area the pressure is also great. As an example, imagine trying to pop a balloon with a needle vs. with a soda can. If you use the same amount of force, the needle will be far more effective.
The unit for force is the Newton (N), so the unit pascal (Pa) for pressure is defined as:
##1Pa=(1N)/(1m^2)##
This is actually a really low pressure, as it turns out. Once we decided on this unit and measured the amount of pressure our atmosphere puts on all of us at the surface of the Earth we decided that kPa would be easier to deal with. Normally, out atmosphere exerts about 101,300 Pa, or 101.3 kPa.
Since nearly all experiments we have ever conducted have taken place on Earth, we’ve simplified pressure further by saying the amount of pressure, on average, measured on the surface of this planet is one atmosphere (1 atm).
The last two units come from Barometers (http://en.wikipedia.org/wiki/Barometer). They mean the same thing, and 1 atm = 101.3 kPa = 760 mmHg = 760 Torr.