The Globus Toolkit is a collection of grid middleware that allows users to run jobs, transfer files, track file replicas, publish information about a grid, and more. All of these facilities share a common security infrastructure called GSI that enables single sign-on. Users can select any subset of the Globus Toolkit to use in building their grid. The VDT includes all of Globus.
Globus has been making a transition from ad-hoc network protcols in Globus versions 1 and 2 (called pre-web services) to web services in Globus versions 3 and 4. You may hear the web services referred to as OGSA, which refers to the high-level architecture, or WSRF, which is the set of Web service specifications that implement OGSA capabilities using web services. Recent versions of the VDT support both the pre-web service and the web services portions of Globus.