Not knowing how much you already know, I'll try to address a few basics without meaning to insult your intelligence.
"Pressure" is pretty much only a concern for people who load (reload) their own ammo. I'm guessing you meant something else. Hopefully I'll catch it below. If not, steer us with another question. Pressure is important to the average shooter in making sure the gun in question can handle the cartridge in question. For example, a "+P" cartridge (such as a .38 Special +P) should only be fired in a gun rated for the extra pressure associated with +P ammo.
Bullet weights: as you know, the bullet is that part of the cartridge that goes down range when the round is fired. Bullet weights are measured in grains. There are 7,000 grains in a pound. A given cartridge can be loaded with bullets of different weights, but there is usually a small number of options that work well for a given cartridge, or which provide enough variation to be worth messing with. For example, the 9mm Luger cartridge (what people usually mean when they say 9mm) comes with a 115 grain bullet, a 124 grain bullet, and a 147 grain bullet; but a 120 grain bullet would be an oddity in a 9mm Luger cartridge.
Muzzle velocity: the size and shape of the cartridge limits the amount of powder it will hold, providing a limit on the max speed at which a bullet leaves the barrel. All else being equal, a light bullet will leave the barrel at a higher rate of speed than a heavy bullet. Muzzle velocity is usually measured in feet per second (fps).
Muzzle energy: the "stopping power" of various cartridges are often compared by measuring their "energy" which is a function of their weight and velocity. The formula is one-half of the bullet's mass times the square of its velocity, resulting in a measure of foot/lbs of force and giving fast bullets an edge ... on paper. There are arguments that larger bullets make up for less theoretical energy by virtue of creating larger wounds.
Barrel length: a barrel that is too short for a given cartridge will allow the bullet to leave the barrel before all the powder has burned, depriving the bullet of some acceleration. A barrel that is too long for a cartridge will slow it down through friction after the powder has provided all the acceleration it is capable of. The ideal barrel length will let the bullet escape after the last measure of acceleration, but before friction reduces its velocity.
Ballistics: air friction causes all bullets to lose velocity. Light bullets shed their velocity faster than heavier bullets. The bullet's shape (long and skinny vs short and fat) also affects the rate at which it slows. The bullet's "ballistic coefficient" is a measure of its ability to hold its velocity. Higher coefficients are "better" than lower coefficients. Obviously, the slower the bullet is going when it hits the target, the less energy it has. For most of us, the ballistics charts published by the ammo makers answer most of our questions.
Here is one example.
I see Pax has already pointed you to her excellent site.
Chuck Hawks' web site is another good source of info but may not have the entry-level information you are looking for. But Bob Spencer has a
good page on basic ballistics.
Hope that helped. Ask away when you are ready.
