Compost? Topsoil?

Jackson and I have been racking our brains as to what the heck is the difference between topsoil and compost. Finally, per email correspondance, someone has given us an answer. Feel free to add to this explanation:

"As per your question, compost-enriched topsoil is not a replacement to compost, but in this case will probably serve you better. While one normally adds compost to topsoil to add nutritive organic matter, topsoil has bits of rock and other grit that creates a firm growing substrate; compost is too soft and nutrient-rich to be used on its own. Most likely, since this soil is of high quality/organic matter, you will not need to use compost with it. (In subsequent years, of course, you will want to add compost as a means of replacing the nutrients lost by growing plants in the soil. This won't be a problem, because you'll have tons of compost from your gardening effort!)
As far as potting soil -- that's just what it sounds like, a lighter, less dense version of topsoil that allows good air flow to plant roots and excellent drainage. If your group is planning to use earthboxes or any kind of containers, I'd suggest picking this up, it can be pricey to buy from nurseries."