'A female friend of another female or of a male' is 'подруга'. The word 'подруга' indicates only friendship and no romantic relationship, just like the English word 'friend'. So one may say 'подруга Маши', one may say 'подруга Антона'. Once in a while people say 'друг' instead of 'подруга', like alena149 mentioned. However, I think that usually when meaning 'подруга', a person had better say 'подруга' since this way of speaking is clearer, but in a humorous phrase it's okay to substitute 'друг' for 'подруга'. Example:
A woman called Lena: Я могу предложить Вам только дружбу. (I can offer you only friendship.)
A man: Отлично, друг Лена! Тогда пойдём по-дружески посидим в кафе на Петровского. (It's perfect, friend Lena! Then let's go out in a friendly way to the cafe on Petrovskogo street.)
'A girlfriend' meaning 'a girl that somebody is having a romantic relationship with' is 'девушка'. To express that a particular девушка is someone's girlfriend, not just a girl, the speaker always says who the girl belongs to, i.e. who is her boyfriend:
девушка того парня.
'A girlfriend' may also mean 'a female friend of another female'. In this sense 'girlfriend' is 'подруга'.
'Невеста' is a bride, i.e. a female on her wedding day or just before or just after it. So 'невеста' and 'a girlfriend' are significantly different.
That is why confusing the words 'подруга', 'девушка', and 'невеста' may well lead to unpleasant consequences