Springtails (Collembola sp.) are soft-bodied insects that live in the soil and leaf litter of terrariums. They assist in completing the eco-system of your tank by aiding in the decomposition of the leaf litter, animal feces, and dead plant material. They also provide frogs with an additional food source.
Glass Box Tropicals offers Temperate, Tropical, and Pink springtails. These are all able to reproduce in your terrarium or vivarium, but many people will also culture them outside of the terrarium in order to have easy access to seed new tanks with them and replenish tanks if need be. Please see the Resource Center article and their care and breeding.