From here: http://agebb.missouri.edu/agforest/archives/v10n2/gh14.htm
Well-drained soil is that which allows water to percolate through it reasonably quickly and not pool. Standing water or saturated soil deprives roots of oxygen. Some tree species can tolerate wet site conditions longer than others, so we make the distinction in our planting recommendations.I actually haven't tested my soil (as it is July and I've just figured out what all the experts mean when they tell me to have 'well drained soil'). I have at least tried to prepare the soil somewhat. For next planting though, I'm going to make sure to do it right. Here is a good article on how to make the illustrious soil if you don't have it.
Deep, loamy soil and sloping sites tend to be well drained. Soil high in clay content, depressions, or sites with high water tables, underlying rock or ‘hard pans’ (a layer of soil impervious to water) tend to not be well drained. A test that is often recommended is to dig a hole 12 by 12 inches square and about 12 to 18 inches deep. Fill it with water and let it drain. Then do it again, but this time clock how long it takes to drain. In well-drained soil the water level will go down at a rate of about 1 inch an hour. A faster rate, such as in sandy soil, may signal potentially dry site conditions; a slower rate is a caution that you either need to provide drainage or look for a species tolerant of wet conditions. Most trees grow best in a deep, moist, well-drained soil. Each species has a different level of tolerance to soils on either the wet or dry side of the ideal. Of course, other site factors such as pH and shade tolerance are also important in considering what trees to plant.
So there you have it folks - happy planting!