It's a question I often get asked when I talk about this research project and my experiences on Shamir. Most people have heard of kibbutzim (the plural form of kibbutz) and have a vague, general idea about them, perhaps from someone they met who lived on one, as communal farms in Israel.
Wikipedia offers a decent basic definition and a detailed overview of the history of the kibbutz movement and recent changes. (Appropriately enough, it is collectively written—Wikipedia is to encyclopedias as kibbutzim are to private communities and farms, although Wikipedia is in ascendence where kibbutzim are on the decline.)
Henry Near, a kibbutznik and author of the definitive two-volume history of The Kibbutz Movement, offers several useful definitions in his book's Glossary:
kibbutz (community): (a) federation of communal groups (plugot, havurot, etc.) and/or settlements (e.g. the Kibbutz Me'uhad). (b) Large communal settlement, combining agriculture with industry, as opposed to the small entirely agricultural kvutza. (c) Comprehensive name for communal settlement.
kvutza (group): (a) Communal working group, whose members contracted to work for a defined time or objective. (b) Small, permanently settled, purely agricultural communal group.
The founders of Degania (later renamed "Degania Alef" to distinguish it from neighbouring Degania Bet) called their community a kvutza. In the early years of the movement, there was great debate about whether these communities were better suited -- philosophically, economically, socially -- to remain small units (ie, kvutza) or grow to be a full-blown kibbutz (some of which count more than 1,000 members).
More recently, with the turn of the millennium, many kibbutzim have voted to reduce the communal obligations of members—paying for food in the dining hall, letting members own their own houses and apartments, and permitting "differential salaries" (ie, market value wages rather than the original socialist concept of "From each according to their ability to each according to their need").
These changes strained the legal definition of what constitutes a kibbutz. In 2002, a national Committee for the Classification of Kibbutzim (known as the Ben-Rafael Committee for its chairperson) met and eventually (after often contentious debates) mapped out a new three-part definition for which communities can call themselves a kibbutz (a designation that has legal and tax advantages in Israel).
There can now be 1) Kibbutz Shitufi -- aka "traditional" or "collective" kibbutzim -- which maintain much of the original cooperative system of collective ownership and redistribution of resources; 2) Kibbutz Mitkhadesh -- aka "renewing" or "innovative" or (often pejoratively) "privatized" kibbutzim -- which have instituted privatization of apartments, differential salaries, and/or distribution of shares of the means of production; and 3) Urban Kibbutzim -- a relatively new phenomenon, of the last 30 years, in which small groups of usually young people live cooperatively in urban settings and tend to be employed in fields of social work and education with a shared vision of social justice.
Some observers say this new flexible definition of "kibbutz" will allow the communities to evolve and thrive, on their own terms, in the 21st century. Other critics told me that these changes have emptied the original concept of meaning and turned it into a "zombie category". Part of the goal in my travels, research and writing is to explore what value and values the concept of the kibbutz maintains in 2010.