From the USDA Definition:
In basic terms, CSA consists of a community of individuals who pledge support to a farm operation so that the farmland becomes, either legally or spiritually, the community’s farm, with the growers and consumers providing mutual support and sharing the risks and benefits of food production.
From PACSAC (Portland Area Community Supported Agriculture Coalition):
A CSA is a combined effort between a farm and a community of supporters (“shareholders” or “members”) that creates a direct relationship between the production and consumption of food. Each season the members provide the money (and sometimes other resources like marketing or bookkeeping services) needed for the farm to operate by purchasing a “share” of the season’s harvest. By making this commitment, a CSA member assumes with the farmer the risks and the rewards of growing the food they will eat.
Members benefit from CSA because, in return for their investment in the farm, they receive a share of the harvest, fresh, healthy, local food each week throughout the growing season. By having a relationship with a specific farm, CSA members also know exactly where their food comes from and how it was grown.
Farmers benefit because CSA is an economically viable way for small scale farmers to produce a wide variety of high quality vegetables, often in an earth-friendly way.
From Farms of Tomorrow Revisited: Community Supported Farms-Farm Supported Communities by Trauger Groh & Steven McFadden:
CSA is not just another new and clever approach to marketing for farmers…community farming is about the necessary renewal of agriculture through its healthy linkage with the human community that depends upon farms and farmers for survival. From experience, we see the potential of community farming as the basis for a renewal of the human relationship with the earth.