There are a variety of substrates available for fish tanks.

You should choose your substrate depending on what you want your tank to look like and how easy you want it to be to set it up. Some substrates are also much more expensive than others. 

Gravel is the most common substrate. It is available in every fish store. It is unnatural and comes in almost every color, from pink to brown to blue. If you are planning to have fish that burrow in the substrate, be sure that the gravel you choose is smooth so it will not rip their scales. Gravel is usually not a good choice for planted aquariums, but some very hardy plants can survive in gravel, especially if you add vitamins to the water or gravel. 

Another option is sand. Sand is available at many fish stores as well. It comes in a few colors, mostly white, tan, or brown. It is relatively easy to clean. You need to stir up your sand every few days because it can compact to create pockets of gas that is dangerous to your fish. Another way to stir up the sand is to keep malasian trumpet snails. They burrow in the sand, but reproduce VERY rapidly. Sand can be more expensive than gravel, and can sometimes be messy or cloud the water. A cheap and clean sand to use is pool filter sand. It is available at any pool store and comes already cleaned. Sand can support a few more types of plants.

A third option is soil, which is created mostly for planted tanks. There are a variety of soil substrates available. Eco-complete and Flourite are popular. Amazonia I Aquasoil is rated as the best, but is also the most expensive, and can get messy when adding it to your tank. Aquasoil also may raise the ph level of the water. 

If you plan on having fish that like to burrow in the substrate, like many species of loaches, you may not want to use a soil substrate. These fish could muddy the tank and possibly uproot your plants.

A good thing NOT to use for most freshwater tanks is crushed corals. This will raise the pH of your tank and hurt your fish. However, if you have a cichlid tank, you should use crushed corals or real coral decorations. These fish need a higher pH than most.