0.40 calibur ammunition, Smith and Wesson, is a calibur of bullet becoming increasingly used by gun manufacturers. It was originally designed to have the stoping power of a 0.45cal ammo, with the smaller size of 9mm. The FBI held a series of tests looking for a new ammunition after it decided the 9mm did not have enough force for it's agents to quickly neutralize a target. It decided on the 10mm, which is a somewhat rare load, with only a few pistol makers such as Glock making pistols which utilize it. The FBI eventually settled on the 0.40cal, Glock 23, as standard issue for it's agents. The new round was more powerfull than 9mm, but was similar enough in size that the size and shape of a pistol can be the same as that of a 9mm and it can still hold sufficient rounds.
Many times a person refering to a 40 cal is actually talking about the gun which uses this ammunition, rather than the ammo itself.