The Fancy Guppy, which most hobbiests favor, is the most popular, breeding and keeping wise though. Guppies of course are live bearers, with a unique internal fertilization system where they can produce up to as many as 12 individual broods of young that develop inside of the mother. That is known as superfetation. All guppies are carnivorous, so if you want the babies, you should get them away from the parents a.s.a.p. and place them in a separate space, which either could be a breeder net or even in an entirly different tank. I recomend a breeder net so that they are accustomed to the environment that they were born into and not shocked by being put into another. By the way, the female can have litters every four weeks and is capable of having 2 to 126 young at a time. If you are wondering how to tell the difference between males and females, it's easy. The males have the brightest and largest fins, and the females baisicly only have color in their tails. You can also tell by the difference of the anal fins.
The aquarium your fancy guppies are in should be kept at a temperature anywhere from 75-85 degrees farenheit (24-29 Celcius) and to encourage breeding, you can raise your water temperature a degree or two from your normal temperature, but don't exceed 85*F. The mortality rate of the guppy is high, and a pair can start breeding at about the age of four months. Guppies are good tank community members and are not agressive to other fish. The guppy diet is easy to follow. They basicly like dry flake foods, and for additional nutrition, you could get them some shrimp pellets, microworms, or even baby brine shrimp. A product with color enhancer, which is usualy strongly advertised on the label, will bring out the colors of your fancy guppy even more.
Guppies are a member of the topminnow family which means that they prefer to feed off of the water surface. The thing that all guppy owners should watch for is fungus. If your guppy is not well, a cottony mass can appear on your guppy's body or mouth. In most cases, it is not life threatening, if treated early. If you catch it when it starts, simply go to a pet store or a department store and pick up a good fungus remover for tropical fish. They are relativly cheap, $1.97's worth can treat about 240 gallons of water. The majority of this information applies to almost all guppies, not just fancies.
The swordtail guppy is a very, very close relative of the fancy guppy. The only difference is that the tail comes to a long extended point on either the bottom, top, or both. Fair warning, they can be a little costly, but if you know a good fish shop that has low prices, ask them to order one for you. Most people think that the swordtail guppy is half guppy and half swordtail, but actualy, it is just a genetic mutation that has been narrowed into a specific strain and given a name as a breed. Males have a more pronounced sword, females baisicly have a stub. A lot of variations on the color are that where some strains are breed to have spots, or almost a solid color, but that is very hard to do. The care for swordtail guppies is the same as fancy guppies. Follow the care information above and they'll be just fine. I would reccommend starting with fancy guppies, and then incorporating swords into your tank. By the way, there have been some male swordtail fish that have been know to breed with fancy guppies, but they are baisicly incompatable, and if something did happen, there is a 0.0001% chance that a baby would live.
The second name, feeder, kind of says it all for this guppy. Most common/feeders are breed for one specific purpose, to be fed to larger fish capable of swallowing them. Common/feeders are much smaller than fancy guppies, about 1/4 or 1/5 size or less. The females have absolutely no color at all and are almost completely transparent. The males have a pastel or more commonly, a neon color mix to them. They are compatable to all other guppies. If a common/feeder would breed to a fancy, the offspring may have a slightly duller color, not be as big, and the tail in the males may be more squared. If you do have common/feeders vs. fancies, watch out because in about the fifth to sixth generation, which can take about a year to reach, could revert back to the form of all common/feeder type fish. I would recomend just sticking with fancies, but if you do put some commons in, put only about one pair to every four to three pairs of fancies. The babies of common/feeders are much, much smaller and harder to preserve. They eat just like all other guppies, and they sometimes scavange from the bottom of the tank. They are friendly and sometimes tend to stay in their own group.
I hope that I have been some help to any one with questions about guppies. There are several resources out there that can provide more help. If you have questions for me, you can contact my link at email@example.com I hope to see some people like me who love guppies!