SPF, or Sun Protection Factor, is meant to measure the protection provided by a sunscreen.  It is defined as the ratio of how long it takes for skin to redden with sunscreen compared to without sunscreen.

UPF, or Ultraviolet Protection Factor, is the ratio of how much UVR (ultra violet ray) is measured at a detector with the protection of a fabric compared to without the protection of a fabric.

Reprinted with permission by New Harbinger Publications, Inc., Sun Protection for Life, Mary Mills Barrow & John F. Barrow www.newharbinger.com