Target shotshells are used to break clays, while field shotshells are used for hunting.
Target loads are generally considered to cover shot sizes 9, 8, 7-1/2 and 7. Your choice of shot size will depend on the sporting regulations you are shooting within, the distance of your target, and the chokes in your shotgun. Due to the smaller shot size and therefore greater number of pellets per ounce, some shooters choose to shoot closer targets with smaller shot such as size 9. This ensures a better chance of connecting with the target. For longer targets a heavier, more massive shot is preferred due to its ability to carry momentum further than the lighter, smaller shot.
Velocity of the shot is also a consideration, though it is less significant than the previously mentioned factors. Shotshells with higher velocities will naturally impose upon the shooter greater amounts of recoil.
The caliber of shotguns is measured in terms of gauge (U.S.) or bore (U.K.). The gauge number is determined by the weight, in fractions of a pound, of a solid sphere of lead with a diameter equal to the inside diameter of the barrel. So, a 10 gauge shotgun nominally should have an inside diameter equal to that of a sphere made from one-tenth of a pound of lead. By far the most common gauges are 12 (0.729 in, 18.5 mm diameter) and 20 (0.614 in, 15.6 mm), although .410 (= 67), 32, 28, 24, 16, and 10 (19.7 mm) gauge also exist.