Depending on the person doing the asking, there are a number of different answers that might make sense to them.
Facebook is for the people you already know and Twitter will help you reach the people you want to know.
Facebook is about who you know and Twitter is about what you know.
Facebook is all inclusive and Twitter is enhanced through third party apps.
Facebook is profitable and Twitter is still looking for a way to monetize.
Facebook is personal and Twitter is professional.
But I think the best way to differentiate...
Facebook is about sharing an emotional connection that encourages engagement (branding) and Twitter is about sharing information.