Powersum NEXT is the NEXT on one pair when all three others are carrying signals. This is realistic with Fast Ethernet and Gigabit Ethernet where all pairs carry signals, often simultaneously. Manufacturers routinely test cables for this characteristic now, but there is no standard yet for the performance of the cable or terminations. FEXT is far end crosstalk, looking at the effect of the coupling from one pair to another over the entire length, measured at the far end. Delay Skew measures how much simultaneous pulses spread out at the far end.
Return loss is a measure of the reflections from the cable due to variations in the impedance.
To get an overview of these proposals, see beyond Cat 5. Wiremappers test the connections and Cat 5 testers test the performance at high frequencies. Some installers use the Cat 5 tester for all testing, after the cable is installed. Others have each crew use an inexpensive wiremapper to make sure connections are correct before the Cat 5 tester is brought in, since it's a very expensive unit that needs a trained operation and many failures are simply wire map problems. By having each crew find and fix their own wiremap problems, testing and corrections are done as the cable is installed and the cost of the Cat 5 tester is not wasted on simple problems.