Recently, a novel inorganic electride stable at room temperatures has been obtained by reducing a complex nanoporous oxide 12CaO center dot 7Al(2)O(3) (C12A7) in a Ca atmosphere (Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. Science 2003, 301, 626). In this system, up to 2.3 x 10(21)/cm(3) electrons can be accommodated in a three-dimensional network of cages formed by a positively charged oxide framework. We demonstrate theoretically that at all concentrations, n(e), the electrons are neither associated with specific atoms nor fully delocalized. At low n(e), the electrons are isolated from each other and resemble the color centers in insulating materials. They are well localized in some of the lattice cages and yield strong inhomogeneous lattice distortions that provide polaron-type cage-to-cage electron hopping. As n(e) increases, the electrons form a denser electron gas and become more evenly spread over all available lattice cages. At sufficiently high n(e), the system becomes metallic but still retains partially localized character of the conducting electrons. We describe the nature of the electronic states at the Fermi level and predict the changes in the optical and magnetic properties of this system as a function of n(e).