One of the main differences is that regular soil is heavier and denser, making it more difficult to use in some flower pots.

Kim's answer is correct. Depending on what brand of potting soil you use, the contents will vary. Sometimes crushed white styrofoam is used to make potting soil lighter. The other item often used to make the soil lighter in higher quality potting soils is perlite. Perlite is natural and has no chemical reactions as an inert, "glass" type substance with a balanced PH of near 7. A volcanic ore is mined, then the ore is rapidly heated to a very high temperature causing the ore to expand like popcorn. The expansion process contributes to the white color, whereas the original ore may have been a glassy dark or gray color. For the most part, perlite contains its silicon [silicon is an abundant element used to make glass (from sands)], but perlite also contains other minerals such as aluminum, potassium, trace minerals, sodium, calcium, magnesium. In horticultural applications, perlite is used throughout the world as a component of soilless growing mixes where it provides aeration and optimum moisture retention for superior plant growth. For rooting cuttings, 100% perlite is used. Studies have shown that outstanding yields are achieved with perlite hydroponic systems. Other benefits of horticultural perlite are its neutral pH and the fact that it is sterile and weed-free. Perlite also acts a a carrier for nutrients. Minerals in the soil, combined with the benefitial microbes and organisms working with carbon based organic materials help to nutritiously feed your plants. Proper nutrition, without synthetic chemicals, make plants healthier (Ha!...just like humans) and help plants to fight off disease or pests. You can make your own potting soil or buy it pre-blended. The big name brands does not mean that it is a healthier soil...you pay for their marketing propaganda. Miracle Grow (owned by Scotts) is a big name brand and is often over-priced with synthetic chemicals added which off-balance the nutrition and health of your plants, but they mask it with "drug doses" of unnatural sources of nitrogen to green up your plant like steroids do in humans.

Both answers have merit. For us laymen, Potting soil will hold more water, which is useful in pots, as they tend to dry out faster than plants in the ground. They usually have time released fertilizers in them too, which are good for 3-6 months. They are lighter and allow roots to grow more easily. Garden soil is dense, compactable, and less air space for roots to thrive. Great for mixing with sandy or clay soils to improve them. Do not use potting soil in your garden. It is light, will float, and has the potential to wash away