Goth chick is short for gothic
It's a reference to a female (usually a teenager) who wears mostly/all black, all the time. She may have experienced extreme hurt in her past and now wants to wear her pain on the outside. Goth chicks are usually open-minded and care either too much or not enough.
Due to the Gothic era and Gothic art, these styles may also be associated with goth chick.
Goth is a fashion and can double as a religion, if you so desire. It's usually a way of life or just style. For most goths, though, it's a form of releasing anger/pain.
There are generally two kinds of goth chicks.
1) Tom boy goth-
A goth chick who wears baggy/tight black pants and band t-shirts or guy t-shirts all the time.
2) Slutty goth-seen most
A goth chick who wears tight black pants and tight shirts all the time. She may also be seen with extreme cleavage or dresses.
Note: Goth is more than a fashion: it's a ways of thinking.