At the risk of getting caught by a punster ... "well draining soil" is soil through which moisture seeps or permeates easily or ie: "well".

Here is a link that might be useful: Well drained soil

When discussing container culture, soils with a high % of small particles hold water tightly in the small spaces between particles. How much water they retain is a direct function of how large or small the particles are. OTOH, soils with a high % of large particles hold less water between the particles. Small particles might be considered 3/32" or smaller. Large particles would be considered 1/8" or larger. Container soils that drain freely will have a large fraction of soil particles >3/32" and a small fraction (15% or less) of particles Another way of looking at it is: A free draining soil is made of particles large enough that most of the water the soil holds is held within the particles and not between them.

Al

A well draining soil is one that allows excess water to flow out fairly quickly and does not get water logged.

You can buy soil at the store, I don't suppose I have ever seen store purchased soil that didn't drain well. As a matter of fact, I have had the opposite problem with store puchased soil, some dries out too quickly. I think that is why most of the people here either make their own mix or compost.

I have purchased topsoil from a mulch company that had a lot of clay in it (Tennessee soil is mostly red clay, I am lucky to not have that problem!!) and it had poor drainage which is why I originally came to gardenweb and lurked about to find out how to improve drainage. I usually judge drainage after a rain. Some parts of my yard will hold water for hours after a rain while others, it drains in a matter of minutes.