Artificial selection is also know as selective breeding. It is where particular traits are selectively bred into a particular animal or crop. This can be done either purposefully or accidentally. Over time and several rounds of breeding, a entirely new species or genetic strain can be created. The pros are probably most obvious with plants and crops. Selective breeding has produced crops over time that grow more fully and produce a larger yield when harvested. They can also grow under adverse conditions (drought, pests, etc.). Selective breeding also has produced organisms that are more aesthetically pleasing to the senses (fruit that looks more colorful, dog breeds that are useful or desirable). The cons are a little more difficult to define. Some believe that artificial selection is akin to "playing God" and usurping nature's plan. On a more practical level, creating a new species or strain can introduce unintended consequences. A new strain of crop might be more susceptible to a particular disease and thus an entire crop could be accidentally wiped out. Or an animal breed might have unforeseen physical complications, thus creating animals that are handicapped with a smaller chance of long term survival.
Artificial selection is the quickest way to produce the traits that one considers desirable. For example, if there is normally a one in four chance of producing offspring with a particular trait, selecting only the embryos that display that trait will increase the odds. The traits that one considers desirable, however, are not always the traits that will increase the chances of an organism's survival. For example, selectively breeding potatoes for uniform size, shape, and taste could unintentionally produce potatoes that are vulnerable to a certain kind of fungus. If all potatoes carry the same defective trait, an entire crop could be lost, which occurred in Ireland during the potato famine.