To put it simply, 'femme' is a descriptor for a queer person who presents and acts in a traditionally feminine manner, as explained by feminist media site Autostraddle. But the differences are precisely why the word femme must exist in the first place. Unfortunately, many folks outside of the queer scene don't fully understand what femme means or recognize its distinct differences from feminine.
But only in the past few years have I called myself 'femme.' The difference stands not in abbreviation, but in queer coding.