Question

What are quantum numbers used to describe?

Answer 1

Quantum numbers are used to describe an atom's electron distributions. Particularly, they represent acceptable solutions to Schrodinger's wave equations for the Hydrogen atom (more here: http://en.wikipedia.org/wiki/Schr%C3%B6dinger_equation )

There are `4` quantum numbers: `n`, `l`, `m_l`, and `m_s`.

The quantum number that indicates the orbital is `l`. This quantum number varies from `0` to `n-1` and idicates the s, p ,d or f subshell:

`l`= `0` -> s subshell;
`l` = `1` -> p subshell;
`l` = `2` -> d subshell;
and so on.

Think of quantum numbers as forming a map to a certain electron's location, each electron having a distinct set of quantum numbers to describe its position.
For example, `n`=`2`, `l`=`1`, `m_l`=`0`, and `m_s`=`+1/2` correspond to one electron located on the second energy level, in one of the three `p`-orbitals (the one corresponding to `m_l`=`0`), that has a positive spin. Therefore, that electron would be located in `2p_y` ( `2p_x` would correspond to `m_l` = `-1` and `2p_z` to `m_l`=`+1` ) and have a positive spin.