An object that can hold, or contain, other objects. For example, an Organizational Unit is a container object because it can contain other objects; it doesn't have to.
Container objects are used as a way to logically organize all other objects in the Directory tree.
See also Object; Creating Container Objects in Chapter 1 of Supervising the Network; Appendix A, NDS Object Classes and Properties, in Guide to NetWare 4 Networks.