Here are some of the basic needs seeds and seedlings have. Gardeners usually add ingredients to customize mixes - like extra peat moss, sand, perlite, etc. More bark and peat and less soil make it lighter. Perlite and vermiculite give it better drainage and aeration. Many contain a wetting agent for uniform water distribution.
Professional mix has the same materials as the premium types but is more finely processed.
It works well for starting seeds or transplanting delicate seedlings. Plant-specific mixes are used when you have a certain plant in mind with special requirements. Orchid, African violet and Cactus are just a few of the special mixes available. All plants need light to grow, but young seedlings will benefit from more intense light than adult plants. You can raise seedlings on a shelf or windowsill in a sunny, south-facing window with good results.