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A pretty decent answer from Wikipedia:
In the United States, standards related to arms and ammunition are maintained and published by the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), which publishes standard internal pressures of calibers, formerly measured in copper units of pressure and currently in psi based on piezoelectric instrumentation. Official +P pressures are established by the SAAMI for certain cartridges; in general the +P pressure is approximately 10% higher than the standard pressure (see chart below). SAAMI does not have a +P+ pressure standard, but this indicates a pressure higher than the +P loading. In both cases this is below the pressure of proof test cartridge, which all firearms are required to withstand before they may be sold. Proof pressure are established by the SAAMI, as a percentage of the working pressure, so this places an upper bound on the +P+ pressures of 30–40%.[4] By way of comparison, magnum calibers may be loaded to nearly twice the pressure of the rounds from which they were derived. Overpressure rounds are commonly defensive rounds and are loaded by police and others in need of maximum power in a compact firearm. Accordingly, most overpressure rounds are hollow points or other types of expanding ammunition.
It must be noted that "higher pressure" is not the same as "high pressure"; +P cartridges are generally loaded to pressures far below those typically found in magnum cartridges. The +P standard is designed so that if a shooter was to accidentally use a +P cartridge in a non-+P-rated firearm, the chance of a one-time explosive failure is minimal as long as the gun was in good physical condition. Repeated firing of +P ammunition in a gun not rated for it will drastically speed mechanical failure of the gun, however, and so it should only be used in firearms designated by the manufacturer as safe for +P use.
[edit] Commercially available +P cartridges
Cartridges that are commonly boosted with +P pressures are the 9 mm Luger, .45 ACP, and .38 Special, which are all cartridges that date from the dawn of the 20th Century. There has been significant improvement in metallurgy and quality since the first guns in those calibers have been made, with the result that higher pressures are now safe in modern firearms. Many models will specify to the degree they can use +P ammunition; for example, many aluminum alloy framed .38 Special revolvers should not regularly be used with +P ammunition, for while the cylinder is capable of withstanding the pressures, the added force will increase wear and reduce the service life of the gun.
SAAMI specifications for common +P cartridges are as follows:
Cartridge Standard pressure +P pressure Notes
9 × 19 mm 35,000 38,500 10% increase
.38 Special 17,000 18,500 9% increase
.45 ACP 21,000 23,000 9.5% increase
.38 Auto 26,500 36,500 38% increase to make .38 Super
.45 Colt 14,000 25,000 79% increase, Ruger only load
The +P+ designation is not currently used by the SAAMI, but is used by some manufacturers to designate loads that exceed the +P SAAMI specification. One source lists the 9 × 19 mm +P+ loading as having a pressure of 42,000 psi, an 18% increase over the standard pressure of 35,000 psi, and the .38 Special +P+ as 22,000, a 29% increase over the standard pressure.[5]
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