Pressure is a force over an area, and therefore there are various units of pressure, depending on whether you use the metric system or not.

The most common unit for pressure in the US is psi, pounds per square inch. The standard international unit is Pascal or Newton per square meter; 15 psi equal about 100 000 Pascal. Then there are some others that come from applications of pressure measurement, such as bar and millibar where one bar (about 15 psi) is the typical atmospheric pressure and Torr is a unit that was mostly used to measure pressure lower than atmospheric pressure, i.e. vacuum.

As far as measuring the pressure inside volleyball, the easiest way to do that is with a "better" pump, which allows you to monitor the pressure (with a simple mechanical gauge) as you inflate the ball. I have a standing pump for my bike tires that does that, for example.