There are basically three types or divisions of tropical fish, freshwater, saltwater and brackish.  If we are speaking of freshwater fish, only, then there are only two types, we need concern ourselves with, livebearers an egg layers.  This page is devoted to livebearers.  

Livebearer fish are tropical fish in which their young are "born alive". This means that the fry remain inside their mother's stomach in the mother's provided egg sack, or embryo, until the fry are fully formed. In other words, the mother fish keeps the eggs inside her stomach until the baby fish are ready to come out. The main species of livebearers that you will find sold at aquarium shops include guppies, swordtails, mollies, and platys. Although there are many more.  Livebearers are originally from Mexico, Guatemala, Honduras, and parts of Costa Rica. Livebearers are native to North America and sold in pet shops all around the country. Its now commercially bred in Florida for profit.
The common Livebearer is an omnivore, eats both plants and animals. In the wild, their basic diet includes the plants that grow at the bottom of rivers and lakes, algae growing on rocks, and tiny water bugs living on the water's surface. In captivity, they mostly eat dried flake food, but do enjoy live food such as live brine. Livebearers don't live in salt water. They live in freshwater, in temperatures from 68 to 81ºF, and pH levels 6.9 to 7.4, but they don't tolerate poor water quality.
Any livebearer is a perfect fish for the beginner aquarist. They're peaceful, can live in all types of aquariums, and they're also quiet easy to breed. Livebearers can have up from 10 to 75 babies at one time. Some can even have babies after their mates die by using saved sperm. The babies then take about to 2 years to develop their colors fully. Most Livebearers are born with one, possibly two, solid colors. Livebearers can have rainbow colors, but the average colors include orange, red, often gray, and black. Special colors may include blue, green, and rarely purple. Some livebearers have transparent fins.
Livebearers average live expectancy is 2 to 4 years. When first born a livebearers size is 1 to 2 millimeters long. It takes about 7 months to a year to grow to their max size of 2 inches. During this time, they’re constantly moving. They almost never sit still and can move very quickly if it feels threatened. You can always tell if something is wrong with the water for livebearers are always the first to die. Especially mollies. But if only a single livebearer is staggering at the bottom it is probably weak or has caught some sort of disease.
When a livebearer is pregnant, the aquarist must separate the mother from the main tank and further yet find a method to keep the newly born fry away from the mom. One of the easiest methods is as follows: Have a tank of 5 to 10 gallons set up. Scatter gravel all over the ground about an inch or so high. The bigger the gravel the better. You should also have well-planted, bushy foliage (plants), but this isn't necessary. Once the fry are born they will seek shelter for the mother will eat them. They'll hide in the dense foliage and swim in between the cracks of the gravel. Once the fry are born, don't disturb the gravel and remove the parent.
Below you are the 4 type of livebearers we have described above, however, one single picture does not show the many variations of developed colors of these fish.

The GUPPY

Scientific Name: Poecilia reticulate Other Names: Millions Fish Family: Poeciliidae Origin: Central America to Brazil Adult Size: 2.5 inches (6 cm) Social: Peaceful Lifespan: 4 years Tank Level: Top, Mid dweller Minimum Tank Size: 5 gallon Diet: Omnivore, eats a variety of foods Breeding: Livebearer Care: Easy pH: 7.0 - 8.5 Hardness: 10-30 dGH Temperature: 64-82 F (18-28 C)

The MOLLY

Scientific Name: Poecilia latipina Family: Poeciliidae Origin: Yucatan, Mexico Adult Size: 4-6 inches (10-15 cm) Social: Peaceful Lifespan: 3 years Tank Level: Top, Mid dweller Minimum Tank Size: 20 gallon Diet: Herbivore, eats algae Breeding: Livebearer, difficult to breed Care: Intermediate pH: 7.5 - 8.5 Hardness: 25-35 dGH Temperature: 75-82 F (25-28 C)

The SWORDTAIL

Scientific Name: Xiphophorus helleri Family: Poeciliidae Origin: Central America Adult Size: 4 inches (10 cm) Social: Peaceful Lifespan: 4 years Tank Level: Top, Mid dweller Minimum Tank Size: 10 gallon Diet: Omnivore, eats most foods Breeding: Livebearer Care: Easy to intermediate pH: 7.0 - 8.3 Hardness: 12-30 dGH Temperature: 64-82 F (18-28 C)

The PLATY

Scientific Name: Xiphophorus maculatus Family: Poeciliidae Origin: Guatemala, Honduras, Mexico Adult Size: 2 inches (5 cm) Social: Peaceful, suitable for community tank Lifespan: 4 years Tank Level: Mid dweller Minimum Tank Size: 10 gallon Diet: Omnivore, eats most foods Breeding: Livebearer Care: Easy pH: 7.0 - 8.2 Hardness: 10-25 dGH Temperature: 64-77 F (18-25 C)