electron shell
Think of an onion, with its layers nested one inside another, the innermost small and tight, the outer ones roomier. An atom's electrons are organised much like that: they group into nested layers around the nucleus, the inner ones close in and tightly bound, the outer ones farther out and held more loosely. Each of these layers is an electron shell.
An electron shell is a group of orbitals that share the same principal quantum number, n, and therefore lie at roughly the same distance and energy from the nucleus. Shells are labelled n = 1, 2, 3, … (or historically K, L, M, …), and each can hold a fixed maximum of electrons: 2 in the first, 8 in the second, 18 in the third, given by 2n². The outermost occupied shell holds the valence electrons that drive chemistry.
Shells explain the layered structure of atoms and the rows of the periodic table at a glance. The caveat is that shells are not hard, sharply bounded layers like an onion's skins; they overlap in space, and within a shell there are finer subshells (s, p, d, f) at slightly different energies — the simple onion is a helpful first picture, not the full story.
Chlorine's 17 electrons fill the first shell with 2, the second with 8, and the third with the remaining 7. That outer shell of 7 — one short of a full octet of 8 — is exactly why chlorine grabs a single electron so eagerly to form a −1 ion.
Nested layers of electrons — the outermost shell drives chemistry.
A shell (same n) is broader than a subshell (same n and same orbital type); a single shell may contain several subshells, such as the third shell's 3s, 3p, and 3d.