Vendors might call their product a potting mix or a potting soil, but there is usually no distinguishing ingredient between them and they tend to be used interchangeably. However—and this is important—some products called "potting soil" are actually soil rather than a traditional potting mix, in which case, they are not a good medium for your potted plants.
The upshot: Choose a product labeled as potting mix over one labeled potting soil, if available, although the ingredients tell the real story.
Now let's explore why.
What Container Plants Need
Container plants and seedlings thrive best when they have plenty of drainage and start out in a sterile environment. Hence, a good potting medium is a "soil-less" combination of peat or coir, pine bark, and vermiculite or perlite, none of which provide any nutrition to plants. This suggests a potting mix over a potting soil, although what's in the bag counts a lot.
Good potting mediums (whatever they are called on the bag) are designed to stay "fluffy" and not compact too much in the challenging environment of containers. A good potting medium won't dry out too quickly but will resist compacting so that the plant's roots can take up water and moisture from their surroundings. The roots must also have access to oxygen, which means that the medium can't be too dense.
Potting soils/mixes also come in different types. There are specific mixtures for succulents, orchids, roses, acid-loving plants and seed starting.
How to Purchase the Right Potting Mix or Soil
Read the bag's ingredients, if listed. If the product contains soil, it's not intended for containers but rather for raised beds or for filling in low spots in a lawn. Some products called "potting soil" do contain perlite or vermiculite or sphagnum moss, but if they also contain soil, they won't be ideal.
Try to find a product that contains no soil at all and is a sterile mix.
If no ingredients are listed, consider the bag's weight. A heavy bag will likely indicate that soil is the main ingredient, so look for light-weight bags. Alternatively, just stay away from bags with no ingredients. If a manufacturer is unwilling to list ingredients, there might be a reason.
Potting Mix Amendments
Some potting mediums contain amendments, including fertilizers or additives that help retain moisture, such as water retaining crystals. If you are growing organically, you probably want to avoid those that list chemical fertilizers as amendments, but a mix might have organic ingredients such as bonemeal, bloodmeal or lime. Remember that, just because a mix has added fertilizer, it is not intended to last the plant through a growing season; you will still need to fertilize and water regularly to ensure the long-lasting health of your container plants.
Vermiculite and perlite are frequent ingredients in potting mixes and are added to create an airier, less-dense environment. Both are naturally occurring, although that's not obvious from their names.