What is the difference between a digital lens and a regular lens for an slr camera?

Answer 1

A lens designated "digital only" has a smaller image circle - just sufficient to cover the sensor.

A lens for a 35mm film camera has a much larger image circle to cover the larger area of a 35mm negative.

If you use a lens designed for a 35mm camera on a digital camera with a smaller sensor, you're actually getting the "sweet spot" of the lens.