In things pertaining to ammunition, weight is measured in a unit called grains. 1 grain is equal to 1/7000th of a pound.
Bullets come in all kinds of different weights because they usually offer different terminal results, one might be more accurate than the other in a particular gun or a certain velocity window is met by one weight of bullet when using a full power load, but not the others. An example of this is 147gr (not 148) sub-sonic ammo. It will not produce a sonic crack and is a good option for use in a suppressed weapon. A heavier bullet will also have more inertia and can penetrate deeper than a light weight (115gr) bullet. I believe bullet design is more of a factor in terminal performance than bullet weight (within reason).
The reason heavier bullets are slower is because more force is required to move that bullet out of the case and down the rifling in the barrel. You can't just put more powder behind that bullet because the added length of the heavier bullet takes up space in the case that could otherwise be used for powder. Another thing that limits velocities with any bullet weight are chamber pressures. More powder will usually mean higher velocities, but when chamber pressures exceed safe levels bad things happen, so chamber pressure is another thing that limits the velocities that bullets can be driven to.
+P means the chamber pressures of that load are higher than the maximum pressure of a standard load. For 9mm, a +P load might be up to 10% higher pressure than a non +P load.
The normal maximum chamber pressure for a standard 9mm load is 35,000 PSI. For a +P load, it is 38,500 PSI. It is this pressure that pushes the bullet down the barrel.
Target rounds are usually FMJ because full metal jacketed bullets are cheaper to make than a jacketed hollow point bullet. FMJs will not cause any feeding issues that might occur with older guns as well because there is no cavity at the front of the bullet going over the feed ramp.
For self defense, carry a quality jacketed hollow point round. A quality expanding bullet increases the size of a wound channel and because frontal surface area is increased, the bullet is limited to how deep it will penetrate. For 9mm, a quality expanding JHP bullet will likely penetrate 10-16" in flesh simulation. FMJs do not expand. They will produce a narrow wound channel and cause excessive penetration (20"+).
Edited: +P can be seen as advantageous because the added pressure increases the downrange velocity of the bullet compared to a standard load. A +P load delivering a 147gr bullet at 1050 feet per second will deliver more energy to your target than a standard load sending a 147gr bullet at 1000 feet per second. The disadvantages could include increased recoil and (insignificantly) increased wear on your firearm. Because +P ammo is generally only defensive ammo, there probably aren't going to be a whole lot of +P rounds going through your gun. As long as you use +P ammo modestly, you should not be overly concerned about it. Most modern handguns are designed to shoot +P ammunition without any problems. Your owner's manual should say what kind of ammo is okay to use. Manuals for most modern 9mm handguns will say +P is okay but rarely do manufacturers say +P+ is allowable. Ruger is one manufacturer that says limited use of +P+ is okay to use in their 9mm pistols with the exception of their LC9.
Really, you could learn a lot more than this with a simple Google search...
9×19mm Parabellum - Wikipedia, the free encyclopedia