The common Jade plant, Crassula ovata, is one member of a large family of plants, the Crassulaceae. Most members are from the old world, although a few are from the Western Hemisphere. This FAQ will discuss the Jade plants, and other similar species can be treated similarly.
Crassulas are generally succulents, and will usually grow a new plant from a single leaf. Few are frost tolerant, although temps. just above freezing will promote bloom. They grow outside only in a few areas of the US and Europe, so the focus here is on Jades as houseplants.
When days grow short and nights are cool, Jade plants in habitat receive most of their yearly rain and also do the majority of their growing. Often times the Jades in the house also react by shedding copious amounts of leaves, leading many people to water their plant. This is to be avoided because the result can be a sudden collapse of the plant. Always allowing the plant to get very dry before watering thoroughly will keep the plant healthy. The dropping of leaves just before new growth begins is normal.

To encourage bloom, allow the plant to go without water around the time of the first frost. When the days get short, withhold the water completely and let the plant withstand the cool nights. Several weeks of this dry, cold treatment followed by regular watering will result in blossoms around the shortest day of the year. Regular watering, or nights too warm, and the plant will remain healthy, but bloomless.

Propagating many of the Crassulas involves detatching a healthy leaf and placing it on suitable soil. Out of the sun, but in bright light, about a month later roots will form. Watering while the plant is forming roots is unnecessary and can be deleterious if the plant starts to rot. Without roots, the water is wasted as the plant has no means to transport it into the leaf. Stems laid or planted in the soil also will root, the only requirement is that they be sound. Rooting is also easiest when days are shortest, Nov/Dec in the Northern Hemisphere.
A yearly cycle for an indoor Jade is usually growth in the early spring, and the plant gets regular water (dilute fertilizer if desired), summer when there is little growth visible and the plant should be allowed to go very dry between waterings. In the fall, around the Equinox, the nights lengthen and cool, and if the plant experiences the cool then water should be withheld, and expect leaf drop as well as some of the branches. If the plant is indoors, it still will likely shed in preparation for the main growing season- the late fall/early winter. When new growth shows (after bloom if the plant flowers), water and fertilizer can be given, but always allow the plant to dry before drenching it thoroughly.