Love begins with a notion; lust, with an image. If you're sexually attracted to someone, it's fairly obvious - you like the way they look. Something about their hair, their face, their ass, legs, penis, breasts, vagina, etc. etc. Sexual attraction means you want to be with that body.

Romantic attraction is all about the person in that body. The way they talk, the things they like and dislike, their interests, their goals, fears, little quirks that make them human. You could envision yourself spending a lifetime with this person outside the bedroom. Romantic attraction means you want to be with that person.

There is, of course, room for bleed and considerable or even complete overlap between those two circles - and the circles move. People get old, life happens. People change their minds and move on from old passions to new interests, life happens. Sometimes you find yourself liking someone you couldn't stand when you were younger. Sometimes, tragically, you grow apart from someone you loved, or they grow apart from you.