The clearest difference is the way the working space is built and accessed. Work trucks can use a pickup bed, flatbed, service body, dump body, or other vocational arrangement behind the cab. A cargo van places its principal load area inside an enclosed body reached through side or rear doors. Those layouts change how material is loaded, protected, organized, and reached.
The categories overlap in purpose: both can be heavily equipped business vehicles, and some truck bodies are enclosed. Compare the actual architecture rather than assuming trucks are always open or vans are always simple delivery vehicles. Body configuration explains much of the practical difference; weight ratings, powertrain, and fitted equipment still need their own comparison.