A cover crop is a plant grown for soil improvement rather than harvest. "Green manure" refers to a cover crop that's turned into the soil while still green. These terms are often used interchangeably, though green manure more specifically implies fresh incorporation.
Cover crops for the home garden serve three distinct functions: nitrogen fixation (legumes only), organic matter addition (all cover crops), and weed suppression and erosion prevention (all cover crops). The benefits are real and documented. The degree of benefit depends on which species you use, when you plant, and how you manage the turn-in.
