It depends on the gender and sexual orientation of the speaker.
A heterosexual woman might use "girlfriend" to refer to a close heterosexual female friend without meaning to imply any sort of romantic involvement. This usage tends to be confusing if the listeners are uncertain of the speaker's sexual orientation.
A heterosexual man or a lesbian would use "girlfriend" to mean a woman with whom s/he is romantically involved. It doesn't necessarily follow that the relationship is exclusive.
Certain gay men use "girlfriend" to refer to any of their close gay friends with whom they are not romantically involved. By contrast, the term for a romantic partner is "boyfriend." Again, it does not follow that the relationship is exclusive.
I suppose a lesbian might hypothetically use "boyfriend" to refer ironically to a straight or gay male with whom she has an affectionate but platonic relationship; however, I haven't actually heard any instances of this.