Just for interest's sake, I'll add something, though Kevinsky has given a comprehensive answer already. The reason you don't just dig up a bit of soil from your garden, put it in a pot and then plant something in it is simple - any pathogens present are now contained in that pot and multiplying, rather than being diluted in open ground, which means there's a risk your potted plant will be killed. Certainly, I've known people do just this, and the plant's been fine, but there is still a risk.
The other thing to note is that some potting composts are loam based, which makes them look more like 'ordinary' soil and which are very heavy when wet. These are the most suitable for large trees and shrubs in pots, so John Innes No. 3 is the one of choice for large, permanent potted subjects - unless the weight is an issue (in a roof garden, for instance).
As a further point, many people can and do make their own potting composts - they use a hot, aerobic composting system for garden waste, and when that is ready, they add other materials (sand, vermiculite, grit, peat, etc) in various ratios to get the potting medium they want for various plants. A hot, aerobic composting system kills off the majority of pathogens and most weed seeds too, which is why that system needs to be used if you want to make your own potting medium.