One way to look at it is this:
Romantic attraction = wanting to date someone.
Sexual attraction = wanting to sleep with someone.
I imagine for most people, those two go hand in hand. They want to go out on dates with a person they like and they want to sleep with them - maybe on the first date, maybe on the seventh, or maybe after marriage (it doesn't matter when they do it, just that the desire to do it is there).
But its not like that for everyone.
For example, some people may be interested in sleeping with an individual they find attractive, but have no interest in an actual on-going relationship with them (e.g. one-night-stands). In this case, they are sexually attracted to the person but not romantically attracted to them.
On the flip side, some people may be interested in going out on dates with someone they like, but have no interest in actually taking them home and sleeping with them (not after any number of dates). In this case, they are romantically attracted to the person but not sexually attracted to them.
Again, that's just my personal interpretation of it, and I may be well off the mark here.