Plus P Ammo
Most shooters see +P and instantly think more power. Although it may be true the +P rating actually stands for "plus pressure" or "increased pressure."
Pressure is what propels the bullet down the barrel. This pressure is regulated by an industry organization known as SAAMI (Sporting Arms and Ammunition Manufacturer's Association).
A cartridge that has been around for a long time, is actually capable of being loaded to higher pressures than SAAMI has established. For example let's consider your .38 Special, a cartridge more than 100 years old. Many currently produced guns in this chambering can handle higher pressures but SAAMI can’t change the set standard because of the older guns in circulation. These older guns can't handle the increase safely.
In this case SAAMI has established a second standard that is refer to as "+P." These are now considered two separate cartridges and must be marked with the +P on the headstamp. The standard .38 Special will read ".38 Special" or ".38 Sp." and the higher pressure rated cartridge is marked with a +P added to its name.
The cartridges SAAMI has established a second, higher pressure, +P version, include the .38 Special, 9mm Luger, .38ACP, and .45ACP. All are old rounds that were created before WWI. Modern counterparts are referred to as .38 Special +P, 9mm +P, .38 Super +P, and .45ACP +P.
The .40S&W doesn’t have a +P counterpart because it was designed in the 1990s, and all guns made for them are capable of firing the established pressure.
The two disadvantages of the +P cartridge are, more recoil and it will be harder on the gun. Even if the gun is rated appropriate for +P ammo, it will wear out faster. The pressure of firing the gun is what wears it out, therefore the higher the pressure the more wear.
Most of us use the +P ammo for personal defense and standard load ammo for practice. After carrying for a while we usually expend the vintage carry ammo and reload with fresh. This gives us the occasional practice with +P ammo that keeps us aware of how it feels to shoot.
Revolvers have less to worry about because they're intrinsically stronger than autos. By design, some gas bleeds out of the gap between the cylinder and forcing cone. When I fire my Magna-ported Ruger NM Blackhawk in low light conditions it looks like a Pink Floyd concert!