I had never heard of "wpi" before but read about it today in my "Talking Crochet" free email newsletter (you can subscribe by going to http://www.crochetmagazine.com/). It's an abbreviation for "wraps per inch." Literally, you determine wpi by wrapping yarn around a ruler for one inch, with each strand of yarn being wrapped parallel and next to each other. Be careful not to wrap the strands of yarn too close to each other or too tightly, no twisting or crossing.
Once you've done the wrapping, you count the number of parallel strands within one inch. Usually, there will be about 16 wpi for fingering weight yarn and about 8 wpi for bulky yarn. This technique is one way of determining the density or weight of a yarn and also of ensuring that different yarns can be substituted for each other in a pattern.