Genitron.com offers a useful comparison chart of common handgun calibers complete with cartridge dimensions and bullet velocities. GunNoob.com also has a handgun ammunition guide with pictures of cartridges and a short description of each. Bass Pro Shops offers a guide to rifle cartridges and discusses their suitability against wild game.
Firearm cartridges are available in three main categories: handgun, rifle and shotgun ammunition. When looking at different calibers, it is helpful to make comparisons within each of these categories. Unlike rifle and handgun ammunition, shotgun shells are measured in gauges. Shotgun shells also contain pellets of varying diameters. A helpful guide to shotgun ammunition is available at ShotgunLife.com.