Loft...height of bristles. Longer for whipping up cream in a bowl(50-55mm), shorter for soaps(46-50mm, better for scraping off some soap from a puck.)
Density...how many bristles are packed into a knot. The more dense, the more expensive, the less floppyand the less it will splay out on the face, the better at face lathering, the more luxurious on the face.
Floppy vs stiff...floppy for whipping creams up in a bowl and "painting" it onto the face, stiffer for face lathering, soap use, exfoliation and scritchy feel on the skin.
"Time flies like an arrow. Fruit flies like a banana." -Groucho Marx