EF means Electronic Focus
L means extra durability and weather sealing
USM means Ultra-Sonic Motor: the Focus motor operates silently
Optics: 8 elements, 6 groups, including one glass molded (GMo) aspherical element.
The lens has a floating design with which the lens adjusts itself to optimize correction as it's focused.
Good News:
1.) Best super-speed 50mm lens I've ever used, with no veiling haze like conventional fast 50mm lenses wide open.
2.) Smaller, less expensive and faster autofocus than the huge 50mm f/1.0 L and much faster autofocus than the 85mm f/1.2L II.
3.) Great, modern ergonomics.
Bad News:
1.) Expensive.
2.) The $125 50/1.8 II is just as sharp if you're not shooting at large apertures.
This lend would be great for portraits.
