At the start of the season you commit to a share from a local farm. The farm uses that commitment to plan their growing season, and in return you get a weekly or bi-weekly box of whatever is fresh and in season. Most shares run from late spring through fall, though some farms offer winter shares too.
The variety changes week to week. Some weeks it is all tomatoes and corn. Some weeks you get something you have never cooked before. That is part of it.