Pressure is the measure of force applied over a certain area. In SI units pressure is measured in newtons per square meter, and refered to as pascals. In english measures the term PSI is used for pounds per square inch.
Pressure is sometimes measured not as an absolute pressure, but as the pressures above atmospheric pressure, also called gauge pressure. An example of this is the air pressure in a tire of a car, which might be said to be thirty PSI, but is actually thirty PSI above atmospheric pressure. In technical work, this would be written as 30 PSIG.