Bond Girls complete the triad of masculinity (violence, gadgets, and sex) that make Bond movies so awesome.
Typically seen at the beginning of the film, these lovely lassies have one thing to do: Bond, James Bond. While their interests vary from good, old-fashioned gambling to setting Bond's collarbone, they lack the guns and/or ejector seats to be of any real use to the plot. In rare instances, Tarts will provide a key piece of exposition (i.e. happening to know where something is or reading a dossier while we stare at Robert Carlyle's holographic noggin), but their key function is to reinforce Bond's status as a playboy.
It makes sense that the women who actually assist Bond, having experienced a long series of near-death experiences, would need a little sexual healing. Evil-girl sex is also perfectly logical as one or both of the parties involved is likely looking for information. However, card players, doctors, and submarine pilots that Bond has known for all of ten minutes are not logical sexual conquests in a world devoid of confused pizza boys and buxom 30-year-old women with semi-legitimate reasons to be wearing schoolgirl uniforms. Bond is, in fact, that good.
Sylvia Trench (Dr. No, From Russia with Love), while technically a Tart, distinguishes herself above all other Bond Girls in two ways. First, she appears in two films; making her the only woman Bond has slept with to have that distinction. Tracy Bond, the one James actually married, was supposed to appear in two films, dying at the beginning of Diamonds are Forever as opposed to the end of On Her Majesty's Secret Service. This idea was scrapped when the powers-that-be realized how much George Lazenby sucked.
Trench is additionally, and more importantly, notable as having inspired Bond's catchphrase. When they first meet, she introduces herself as "Trench, Sylvia Trench", which Bond copies.
Ah, bad girls. There are few things in this world sexier than a slim yet buxom woman who is planning on killing you afterwards. (Or during, in Famke Jansen's case.) While not as diverse in occupations as the Tarts, Bond films just wouldn't be the same without them. After all, how are we supposed to believe that Gert Frobe is actually going to become the leader of the entire planet if he can't get even one hot girl to do his bidding?
The truly great Fatales are those for whom sex and killing go together like martinis and baccarat. The queen of all Fatales is the shockingly non-innuendo named May Day. A steroid-infused assassin and wanton sex machine at the beck and call of Christopher Walken. The early Bond films emphasized the Girlfriends, with Fatales usually having small roles (though still pretty badass; i.e. Bambi and Thumper from Diamonds are Forever); but the some of the later films produced Fatales much more memorable than their hard-to-get counterparts. Goldeneye's Xenia Onatopp nearly dethroned May Day as Queen Fatale but, as appealing a death as crushed-by-Famke-Jansen's-thighs is, there's something to be said for the possibility of seconds.
Okay, while old, fat Russian guys need hot assistants to convince us of their power, Sean Connery could be completely celibate and still save the world without anyone in the audience questioning it.
In the early Bond films, the Girlfriends aren't much better than the Tarts. They give Bond someone to explain the plot to and provide an excuse for innuendo. Later, sissier (read: less Scottish) Bonds, however, needed someone gorgeous to convince us that the smarmy, middle-aged Londoner is actually quite charming.
The Girlfriends also tend to be the big names, girl-wise, for the films. These include legendary beauties (read: Playboy models) Ursula Andress, Claudine Auger, Barbara Bach, Maryam D'abo, and Denise Richards. Putting down the skin mag for a second, some of these women have been legitimately good actresses. Diana Rigg, OHMSS's ill-fated Bond bride, later won a Tony and an Emmy (which apparently are some kind of award they give out on not-the-internet). Michelle Yeoh, TND's hard-to-get lady spy, is most frequently seen as the edibly Oedipal counterpart to Zhang Ziyi in movies that win Oscars. Jane Seymore, L&LD's ingenue tarot reader, later played the randy, enhanced mom in Wedding Crashers but also was a family friend of Johnny Cash and married Admiral fucking Ackbar! (The marriage only lasted a year because, reportedly, it was a trap.)