A terrarium is essentially a small, enclosed ecosystem, usually just consisting of plants and soil. There are many different kinds of terrariums you can make, and they can often brighten up a room if you don't have much space for a houseplant or garden. I've used them in my dormitory at college before, which is an especially good place for them.
Once you water a terrarium, you don't have to do it again, because water evaporates within the closed system and falls back down to water the plants again. This is especially good for a college environment, since it's easier to leave things like terrariums in the dorm over breaks rather than take them home.
One of the easiest terrariums to make is a moss terrarium, and it might not even cost you anything if you already have a large jar or other glass container lying around. All you need for a moss container is a layer of soil at the bottom, although there doesn't need to be very much soil. Moss is able to grow on rocks a lot of the time, so it doesn't have a very deep root system. Speaking of rocks, it's a good idea to add some of them, too. It will make the terrarium more aesthetically appealing, and the moss can grow on top of the rocks if it wants to.
If you live in an area like Illinois, it's likely that there is moss somewhere outside, probably in your own back yard. You can just take that moss from your yard and put it on the soil in your terrarium. Soon, it will anchor to the soil and rocks and start to grow. Now you've got yourself a moss terrarium (but make sure to water it once, and then close it!).
Different ways to jazz up your terrarium
You can also put other kinds of plants in terrariums. Tropical plants are popular, because they enjoy a very humid environment. Tropical plants will generally do fine in normal soil.
Desert plants are popular for terrariums, too, although you probably shouldn't water them as much. They also require sandy soil, since that's what they're used to in the wild. One important thing to remember, especially if you have desert plants, is to place a layer of rocks or pebbles underneath the soil in the terrarium. This allows the soil to drain, preventing root rot from occurring. I learned this the hard way, so I will always remember to put rocks in my terrarium now.
You can accent terrariums with little statues, and with a moss terrarium, you could create a mini prehistoric scene with plastic dinosaurs. Get creative. I love the fact that there is a lot of versatility with terrariums; no matter your tastes, you'll enjoy making one, too!