I think the fundamental difference is one is to do with innate features, the other is to do with sex-appeal.
I think "prettiness" is to do with innate features (e.g. nice eyes, good facial symmetry, a nice smile, cheekbones, button nose etc. are all things that make you classically "pretty)". On the other hand, attractiveness is more about how you choose to dress, choice of makeup, how you carry yourself, self confidence etc. Moreover, as words like "hot"/ "fit" have sexual connotations, I would say it's more about the body type has a significant influence (dare I say it... tits and ass lol)
IMHO, you're either pretty or you're not... and only a fat wad of cash and a bloody good surgeon can make any real difference. On the other hand, anyone can make themselves attractive by choosing how they dress, carry themselves, being self confident etc.... Even if you don't have it naturally, its something you can work on in one way or another. Furthermore, anyone can increase their sex-appeal by doing things like going to the gym, learning to dance etc.
For example, as a red-blooded hetrosexual male, I would say people like Nicole Scherzinger Cheryl Frenandez-Versini (or whatever the f**k she calls herself now) are both classically "pretty" but I don't really want to jump them (if you know what I mean). On the other hand, girls like Lady Gaga or Britney Spears (in her day) are not as classically pretty as the previous examples, but there's something about them that makes me want to rip their clothes off (in a non- rapey way of course lol)
Hope that helps