There are two main sources of air pressure for aeroplanes:

Static pressure (or hydrostatic pressure) is experienced when submerged in any fluid, be it air or water. It is a result of the weight of fluid above the point at which it is experienced. This pressure acts equally in all directions, for a given depth. On the surface of the Earth, we experience atmospheric pressure as a result of the weight of the atmosphere. As height increases, static pressure decreases as there is less atmosphere above you.

Dynamic pressure is an additional pressure experienced as a result of fluid flow. You can experience this pressure if you put your hand out the window in a moving car - this pressure will push your hand backwards - so it does not act equally in all directions. It is due to the movement of air, or the movement of an aircraft through the air and is used to produce lift. It also creates drag. The equation for calculating dynamic pressure is:

P_D = ½ρV²
Where ρ is the air density, and V is the relative speed.

Total pressure is the sum (combined effect) of static and dynamic pressure. This is what a pitot tube measures, and is also called the "pitot" or "stagnation" pressure.