What is Shells, subshells and electron configuration?

The electrons of an atom are arranged in shells, numbered $n = 1, 2, 3, \ldots$ from the nucleus outwards; the higher $n$, the further from the nucleus and the less tightly held the electrons. Each shell is divided into subshells named by a letter: shell 1 has an s subshell (1s), shell 2 an s and a p subshell (2s, 2p), shell 3 has 3s and 3p (and 3d, met later). An s subshell holds at most 2 electrons, a p subshell at most 6. The electron configuration of an atom lists its occupied subshells with their numbers of electrons as exponents: $1\mathrm{s}^2\,2\mathrm{s}^2\,2\mathrm{p}^4$ for oxygen.