"Girlfriend" and "boyfriend" are most usually conveyed in Russian as девушка иand парень (or молодой человек).
Подруга usually means "girlfriend" as well but its meaning is somewhat less suggestive of a romantic relationship than that of девушка.
As a male you do not call your female friend подруга, not without raising an eyebrow. Women, on the other hand, could refer their female friends as моя подруга (while using моя девушка would unequivocally imply romantic relationship with her).
If you want to rule out any romantic component in describing your friendship, you should use знакомая ("acquaintance") or приятельница (female equivalent of "buddy").