Leaf mould is literally composted leaves, resulting in a rich brown/black organic compost. It takes at least a year to compost down the leaves in order to turn it into leafmould, depending on leaf-type: beech or oak leaves decompose readily, but waxy leaves take much longer. (Sycamore and London Plane in particular take years to rot down) Also warm, wet weather conditions promote faster leaf decomposition and a more readily usable leafmould compost
Once the leaf mould is fully decayed, there should be no whole leaves present - just a crumbly brown/black compost which can be put through a coarse sieve to remove twigs etc. The leaf mould can be added to potting compost (1 part leaf mould to 4 parts potting compost), or used liberally in the garden as a plant mulch, or dug in to improve soil structure.
Leaf mould is quite high in nutrients. However, the main benefits of using leaf mould are derived from the fact that it is an excellent soil conditioner, and has beneficial fungi within its structure. It will provide food to enable the micro bacteria in your soil to carry out their task of enabling plant life to live.
Most plants in the wild natural state live off leaf mould - in one form or another. It is an integral part of the food cycle in the world's natural forests.
When applied to garden soils as a mulch, the main nutritional effects are after the leaf mould has been taken down into the soil by various forms of life - notably the earth worm population. Once it is incorporated into the soil - and also before a a mulch - the soil microbes break the leafmould down into various nutrients which are then available as plant foods. Generally, for garden soils, leaf mould from broad leaf trees and shrubs are better than leaf litter or leaf mould that originated in conifer woodland.
1. See also Organic Fertilizers.
2. Mulching in the garden - Very few operations make a garden instantly more attractive than a good mulch.