A terrarium is a type of miniature ecosystem of plants. Terrariums are usually sealable glass containers that can be opened for maintenance and to access the plants inside. However, this is not essential, terrariums can also be made using other transparent materials and some are open to the atmosphere rather than being sealed. Terrariums are often kept as decorative or ornamental items in the same way as aquariums.
Closed terrariums create a unique environment for plant growth, the transparent walls allow for both heat and light to enter the terrarium. The sealed container combined with the heat entering the terrarium allows for the creation of a small scale water cycle. This happens because moisture from both the soil and plants evaporates in the elevated temperatures inside the terrarium. This water vapour then condenses on the walls of the container, and eventually falls back to the plants and soil below. This contributes to creating an ideal environment for growing plants due to the constant supply of water, thereby preventing the plants from becoming over dry. In addition to this, the light that passes through the transparent material of the terrarium allows for the plants within to photosynthesise, an important aspect of plant growth.
The first terrarium was developed by the botanist Nathaniel Bagshaw Ward in 1842. Ward had interest in observing insect behaviour and accidentally left one of the jars unattended. A fern seed grew, germinated into a plant, and the terrarium was born. The trend quickly spread in the Victorian Era amongst the English. Instead of the terrarium, it was known as the Wardian Case.
Because of the different conditions within, terrariums can be classified into two types: closed and open.
Tropical plant varieties, such as mosses, orchids, ferns and air plants, are generally kept within closed terrariums due to the conditions being similar to the humid and sheltered environment of the tropics.
Keeping the terrarium sealed allows for the circulation of water, but terrariums must be opened once a week to remove excess moisture from the air and walls of the container. This is done to prevent growth of mould which could damage the plants and discolour the sides of the terrarium, this contradicts the myth that terrariums are never opened.  Terrariums must also be watered occasionally, the absence of condensation on the walls of the terrarium or any wilting of the plants is an indicator that the terrarium requires water. 
Closed terrariums also require a special soil mix to ensure both good growing conditions and to reduce the risks of microbial damage. A common medium used is peat-lite a mixture of peat moss, vermiculite and perlite.  The mixture must be sterile in order to avoid introducing potentially harmful microbes. 
Open terrariums are better suited to dry plants such as succulents. Not all plants require or are suited to the moist environment of closed terrariums. So for plants adapted to dry climates, open, unsealed, terrariums are used to keep the air in the terrarium free from excess moisture.