There are many different definitions of work teams and groups used by both academicians and practitioners.  Most of the commonly used definitions have these three elements:

1) It's a "real", intact group, complete with boundaries, interdependence, and differentiated member roles.  Members are dependent upon one another for some shared purpose.

2) There are one or more tasks to perform.  The team produces some outcome for which members have collective responsibility.  Additionally, that outcome can theoretically be evaluated.  The type of outcome (e.g. product Vs. service) is not important.

3) The team operates in an organizational context.  This means that the team, as a collective, manages relations with other individuals or groups in the larger social system in which the team operates.

Again, remember that there are other definitions, but most of them have some combination of the above elements.