Here are the basics of a CSA according to Local Harvest: a farmer offers a certain number of "shares" to the public. Typically the share consists of vegetables, but other farm products may be included like fruit, grains or dairy. Interested consumers purchase a share (aka a "membership" or "subscription") at the beginning of the season and in return receive a box (bag, basket) of seasonal produce each week throughout the farming season.
The CSA I belong to, Katchkie Farm, is an organic farm about 3 hours north of New York City. For $275, I receive a half share of vegetables every Wednesday from June through November. I get the same selection of vegetables as those members that purchased a full share, but I just get half as much. For 2 of us, this is more than enough. I also joined a fruit share and a bread share through Local Roots NYC.
Here are pictures of some of the fresh vegtables, fruits and breads I have received so far this season and some of the dishes I have made.
If you are interested in joining a CSA, here are some helpful links: