An electron shell can be thought of as a spherical shell-shaped region around the nucleus of an atom where electrons can orbit, like a planet's orbit. (This is merely a convenient model: orbits are more varied and complex than that.) Due to the quantum nature of angular momentum (angular momentum can only take certain values which come into play at small distances and masses), electrons cannot orbit just anywhere around a nucleus and the set of possible orbits of a certain angular momentum value are termed shells. Conceptually, around a nucleus is a smallest shell, then a surrounding larger shell, and another, and so on, and each such shell has a maximum number of electrons that can occupy it. Associated with each shell is an electron quantum number termed the principal quantum number (or radial quantum number), often listed as n, which can be thought of as numbering the shells outward. X-ray terminology typically uses a letter to indicate the shell, K for n=1, L for n=2, etc., for example, the term K-line for iron spectral lines settling to shell n=1.
|shell's n||shell letter||limit on electrons within the shell||limit on electrons up to the shell|
The actual maximum number of electrons within a shell for an atom in the ground state (lowest energy state) is also dependent upon quantum-mechanical restrictions. An additional concept, electron subshells, which are subdivisions of the shells, is used to outline rules for determining much of the consequences, such as that the outermost occupied shell generally has eight electrons at most, and there happens to be no instance (among known elements) of any shell occupied by more than 32 electrons.
Electrons can occupy a higher shell than they need to, in which case the atom has a higher-than-minimal energy (atomic excitation), and an electron may spontaneously shift to the lower shell, the lost energy going into an emitted photon, and conversely, an incoming photon can lift an electron from its shell to a "higher" one. Between any two shells is a specific energy-difference, and the tendency of certain frequency photons to be emitted or absorbed by an atom is due to these energy differences. The effect of the same transition among many such atoms is the mechanism behind spectral lines.
Molecules have more complexity in electron orbits.
The electron shell concept is used in chemistry, to explain certain elements' tendency to interact, and in spectroscopy, to help explain and interpret spectral lines.