A terrarium can be made creatively in a way you want. There is no hard and fast rule or necessities that are absolutely required. Yes, but the basics are the plants and a glass jar and some soil media. A Terrarium can be custom built as per your requirement. 

Some of the things you need are as follows: 

1. A glass or plastic container. Glass is always preferable.
2. Rocks, if you choose. But recommended for creating a false bottom.
3. Moss, if you choose.
4. Soil (growing medium). A garden soil for direct use in terrarium is usually not recommended. 
5. Plants that won’t overgrow (generally miniature or dwarf plants)
6. Spoon for placing soil.
7. Long tweezers for putting materials into vessel. Also called as Aquascaping tweezers 
8. A net to separate false bottom layer from soil
9. Spray bottle or syringe to water the terrarium. 
10. Decorative miniatures
11. River sand (for decoration) 
12. Pebbles (for decoration)
13. Charcoal for keeping the terrarium free of toxins.