It is billions of individuals that live in the soil who do the work. Worms are the heavyweights that we are all familiar with. In one end goes rotting organic material, and out the other comes castings, or the purest form of compost. It is sterile, FULL of plant nutrients, and completely odorless. It looks like soil.
Plants love it, and use it as fertilizer.
The more worms you have in your soil, the better. There are also billions of microscopic creatures that, by feeding off rotting organic materials, either give off beneficial waste products, or alter the material to where it becomes food for yet another creature in this giant digestive process. When you feed your soil, your soil feeds you. Treats like compost, leaves, manure, different organic mulches, all contibute to a healthy balance of critters in your soil. Encourage garden worms by mulching your bare soil or planting a cover crop to keep them working and multiplying.