Anything that will hold soil can be used as a garden container. Additionally, a good container should resist rot and corrosion. Unless you're growing water plants, the container should also have a drainage hole. Stylewise, the sky's the limit. Terra-cotta is a timeless classic, but it tends to dry out more quickly than other containers.
Plastic is a hard-working, inexpensive alternative.
Often, it takes a backseat in the looks department, though, so you might have to doctor it up a bit. Cover the drainage hole of the pot with a coffee filter to prevent the soil mix from escaping, then fill the container to within 2 in. Forget conventional spacing recommendations and cram plants together. Start from the middle and work outward. It can be a challenge deciding what to plant.