Soil is a mixture made from minerals, organic matter, liquid and air. The size of soil particles is referred to as its structure. When the particles are large---sandy soil---water drains through easily, but takes nutrients with it. The smallest particles produce clay soil, which retains nutrients, but also a lot of water, which can cause root rot. Such garden soils present problems for potted plants. Instead of garden soil, container plants use potting soil, or, potting media.
Commercially available potting soil comes sterilized so you don't have to worry about insect eggs hatching and infesting your plant or diseases that will sicken or kill your specimen.
Besides standard, all-purpose potting soil, you can choose from different formulations, that is, you can buy potting soil geared toward a particular plant type. There are potting soils made especially for tropical plants, African violets, orchids and other kinds of indoor plants.
Absence of Seeds
Commercial potting soil contains no grass or weed seeds, saving you from having to weed potted plants. The presence of seeds is one reason why using homemade compost instead of potting soil to grow indoor plants, despite the compost's nutrients, isn't advisable. (To use your compost, you'd have to first bake it in your oven, then, after you've pasteurized it, add other materials.)
Potting soil can be easily customized at home. For instance, if you think the soil you've bought is too dense for a plant's needs, adding perlite to the mixture will increase the medium's drainage.
Make at Home
You can't order up instant garden soil, but you can start indoor plants in a homemade potting mix that suits your requirements. Typically, potting media include some mixture of peat moss plus washed sand, perlite and/or vermiculite to which you add fertilizer.
Mixture is Rich in Nutrients
If you buy a good potting soil mix, it will likely contain fertilizers already that will release slowly into the soil for a few months, giving your newly potted plant a good start. Read the packaging.