In simplest terms, pressure is described as the ratio of applied force (perpendicular to an area) to the surface area over which the force is applied.
i.e, pressure = force/surface area
or, P = F/A
where, P is the pressure, F is the normal force and A is the surface area.
The units of pressure are N/m^2 (or any other area unit, such as mm^2, cm^2, etc.), Pascal, psi (pound per square inch, traditional unit), atmosphere. Pressure may also be expressed in terms of height of a particular fluid (such as water or mercury) in a manometer. There are a number of other units of pressure as well.
Just a small note, Pressure (of a liquid) can also be measured in terms of height of fluid as,
P = `rho g h`
i.e. as the product of density of fluid, acceleration due to gravity and height to which it rises.