When you inflate a balloon or a tire, you are increasing the pressure on the inside of the object in order to "blow it up". Pressure is a scientific concept that applies to gases and liquids. Pressure is a force exerted ("spread out") over an area. In a balloon or a tire, pressure is the force that pushes outward, stretching the rubber. Pressure is measured in terms of force per unit area. In the English system of units, pounds per square inch (psi) is one common unit for pressure. In metric units (the SI system), the pascal (Pa), which is equivalent to newtons per square meter (N/m2), is often used to measure pressure.
We use the concept of pressure in many situations when we talk about gases and liquids. When you pump up a tire on a car or bicycle, you use a pressure gage to decide how much compressed air the tire can safely hold. When you dive to the bottom of a swimming pool, you feel the pressure of the water (which gets stronger as you go deeper) squeezing in on your ears.
Earth's atmosphere has pressure. Atmospheric pressure is higher at sea level, and decreases as you go higher up in the atmosphere. Some weather systems have slightly higher pressure than others; weather forecasters often talk about "high pressure" and "low pressure" weather systems. Water also exerts pressure. A submarine has to have a strong hull to withstand the crushing pressure exerted by water when it is deep beneath the sea.
What causes pressure? Atoms and molecules in gases and liquids are constantly in motion, rapidly flying and bouncing and jostling about. They collide with each other, and also bounce off the walls of whatever container they are enclosed in. Whenever they hit the wall of a container (whether a balloon or tire or a SCUBA tank), they exert a tiny amount of force as they bounce off that wall. Summed up over millions of atoms or molecules, this force is what we experience as pressure. Imagine millions of microscopic balls bouncing around a room and off its walls at high speeds; that should give you a pretty good idea of how pressure works. The force of pressure is always pressing in a direction perpendicular to the "walls" of the container.
There are many different units used to measure pressure. Several of these are obsolete, but are still used in certain fields for historical reasons. As was mentioned earlier, pounds per square inch (abbreviated as p.s.i.) is common in the English system of units, and the pascal (abbreviated Pa) is the standard in the Metric (SI) system. Since the pressure exerted by Earth's atmosphere is of great practical importance, pressure is sometimes expressed in terms of "atmospheres" (abbreviated atm). At sea level on Earth the pressure is 1 atm; at a depth of roughly ten meters beneath the oceans it rises to about 2 atm; and on the surface of the planet Venus atmospheric pressure is a crushing 91 atm! Since one atmosphere of pressure is equal to 101,325 pascals, you'll often see the kilopascal (kPa) used in actual measurements; atmospheric pressure is therefore just slightly greater than 100 kPa.
Meteorologists frequently use the bar and millibar (mbar or mb) when describing pressure associated with the atmosphere and weather phenomena. One bar is 100,000 Pa, or almost 1 atmosphere of pressure. A millibar is one thousandth of a bar, so 1 atm is 1,013.25 mbar. You'll often hear millibar used by meteorologists when describing low or high pressure weather systems or the low-pressure center of a hurricane.
In the past, pressure was often measured using an instrument called a manometer. The pressure of the air would force liquid mercury up a tube; scientists would measure how high up the tube the mercury flowed, and report the pressure in terms of inches of mercury or millimeters of mercury. Hg is the standard abbreviation for the element mercury. So, even today, you may sometimes see pressure reported in terms of "mm of Hg" or "inches of Hg". For example, when a doctor or nurse measures your blood pressure, they will usually report the result in terms of mm of Hg. One atmosphere of pressure is equal to 760 mm of Hg. An obsolete unit called the "torr" is sometimes used as a shorthand way of saying "mm of Hg".
When you measure the pressure in a tire with a tire pressure gauge, you are actually reading the pressure in the tire above and beyond atmospheric pressure. This is called, not surprisingly, gage pressure. If your gage says your tire's pressure is 60 p.s.i., for example, it means the pressure is 60 pounds per square inch higher than atmospheric pressure. The actual total pressure in the tire is 60 p.s.i. plus one atmosphere (about 14.7 p.s.i.), or 74.7 p.s.i.
Water (or other fluids) in pipes and other plumbing have pressure. Pressure plus speed plus height of fluids in a plumbing system are mostly conserved; some energy is lost to friction. A pump can be used to increase the pressure of a fluid in a pipe. This pressure can be converted to height as the fluid is forced uphill; or it can be converted to speed as the fluid is made to move and flow along. This is why water storage towers are usually erected on the tops of hills; as the water flows downhill from the tower to your home, it gains pressure which can be used to make it flow back up to eventually squirt out of an upstairs shower or faucet.