Shells and subshell
A subshell is a subdivision of electron shellsseparated by electron orbitals. One or more orbitals in the electron shell of an atom with the same energy level. Subshells have different shapes and are distinguished by their magnetic quantum number. Subshells are labelled s, p, d, and f in an electron configuration.
• All electrons that have the same value for $n$ (the principle quantum number) are in the same shell
• Within a shell (same $n$), all electrons that share the same $l$ (the angular momentum quantum number, or orbital shape) are in the same sub-shell
• When electrons share the same $n$, $l$, and $m_l$, we say they are in the same orbital(they have the same energy level, shape, and orientation).
So to summarize:
• same $n$ - shell
• same $n$ and $l$ - sub-shell
• same $n$, $l$, and $m_l$ - orbital