I just got back from visiting some friends in Lyon and I asked them to explain how to talk about friends versus boyfriends/girlfriends in French; something I've never been sure about.
In school I was taught that:
un ami - a male friend
une amie - a female friend
un petit ami - a boyfriend
une petite amie - a girlfriend
then there is the more casual vocabulary:
un copain - a male friend
une copine - a female friend
What has often confused me though is that ami/amie and copain/copine can also mean boyfriend/girlfriend as well as just friend. An important difference when discussing your friends or love life with someone French.
My friends explained it like so - expressions such as:
J'ai une copine qui s'appelle Jeanne
Jeanne, une de mes amies
suggest friends, whereas using the possessive suggests more than friends:
Jeanne, ma copine (girlfriend)
Robert, mon copain (boyfriend)
Having said that, while petit ami and petite amie definitely mean boy and girlfriend, the words ami/amie/copain/copine can often be ambiguous and there are no absolute rules; it all depends on the context.
Not simple all in all..
Gosh, no wonder the French have such complicated love lives... ;-)