CSA stands for "Community Supported Agriculture. The way a CSA generally works is that you sign up and pay the farmer for a weekly share of the crops from around June to October. Sometimes there is also a work commitment. At Stearns Farm (which I belonged to in years past), I was required to work 12 hours over the growing season. This entailed planting crops, pulling weeds, haresting, etc. I actually liked this part because it really connected me to the farm and I could walk through the farm and say "I planted that Kale and those tomatoes". This year I joined Lands Sake Farm which I don't think requires a work commitment. It is a little closer to my office and will be easier to pick up each week.
CSAs can be expensive, but you get a ton of local organic vegetables every week (most CSA farms are organic). If the rates seem high to you, you can sometimes purchase a 1/2 share or find a friend and split a share. Generally, when splitting a share, you would pick up every other week. In addition to getting great local organic vegetables, you get fresh picked, minimally transported vegetables (your car)... thus also helping the environment!
You can find a CSA near you, by checking out this web site. You will need to sign up right away as it is already getting late in the sign up season and some farms will already be sold out for the season. If you live in the MetroWest area of Boston, Lands Sake farm still has some open shares.