C.S.A. stands for Community Supported Agriculture.
C.S.A. is a direct relationship between people who grow & produce food and those who eat it, driving down costs for both. C.S.A. keeps food local, therefore fresher. It makes packing and shipping obsolete, saving fuel and resources. It keeps money in the local economy, and improves a community's food security.
"Share holders" generally buy a share directly from farmers before the growing season, and receive fresh produce throughout the season. They share in the risk of a failed crop, but far more often, in the abundance of truly fresh and beautiful food.
Here is a link (to a far away C.S.A.) that explains the pros and cons of joining well.
Here is more information from Local Harvest about how it all works.
OK, so what on earth is winter C.S.A.?