"Agartha" and "Agharti" are Hindu words for the Underworld. Nowadays the name Agartha is often used as the name of an ancient civilization that lives underground.
Many people believe there is a network of over 100 underground cities, including a city under Mt. Shasta, CA, that are connected with each other. Most of these cities are located underneath the energy centers of Earth. Some sources mention an underground community under Mt. Shasta that is called Telos (so its inhabitants would be called the Telosians). The capital of Agartha is called Shamballa (or Shamballa, The Lesser), named after the city of the Ascended Masters.
The inhabitants of Agartha are said to be a benevolent human species, that have technological capabilities that exceed ours by far. E.g., they have a space fleet that is called the 'Silver Fleet.'
The Agartha Network should not be confused with another underground network of (military) bases that allegedly is inhabited by 'Lizzies,' Greys, Members of the Draconian Empire, and Secret Government people, with whom the inhabitants of Agartha are said to have been in conflict for a long time. They even lost some of their cities to them.
Some of the major cities of the Agartha network, apart from Shamballa and Telos, are:

Both the RA Material and Alex Collier claim the some of the Agarthians originally came Maldek (now the asteroid belt). Collier refers to them as the lost tribe of Lyra, who were also called Ari-an.
The RA Material, as well as remote viewing sessions by the Farsight Institute refer to refugees from Mars who also live in this underground network of cities.