You can also go the no-substrate route, but that limits your plant options.
_Substrates:_There are tons of substrates on the market, but the most common are gravel; specialty planted tank substrates like Fluorite, Eco Complete, or Fluval Stratum; sand; and dirt. There are advantages and disadvantages to each. You can also go the no-substrate route, but that limits your plant options. Even if you weren't placing plants, though, substrate is useful for providing surface area to grow benefifcial bacteria and gives your tank a more natural look.