In the world of sustainable agriculture, CSA stands for Community Supported Agriculture.
In its simplest form, this is an arrangement between a farmer and a consumer seeking fresh produce. Members receive weekly shares of fresh natural produce at a reasonable cost, and the farmer receives an income to cover farm expenses and labor.
With a CSA patrons are usually asked to pay a membership fee up-front. This allows the farmer to carefully plan, purchase seeds and supplies, and make needed farm improvements. By selling farm memberships directly to community members, farmers gain some sense of financial security and are relieved of much of the burden of further sales and marketing. In return, members receive weekly shares of the farm's bounty throughout the growing season (food that is fresh, locally grown, and far better tasting than what can typically be purchased at a grocery store), as well as satisfaction gained from reconnecting to the land and participating in a sustainable food production system.
Another CSA principle is that the farmer and the consumer share both the inherent bounty and/or risk associated with producing locally grown organic produce, including poor harvests due to unfavorable weather or pests.