A woodworker’s workbench isn’t a table, it’s a work-holding system. It’s not something you set things on top of, it’s a tool that holds your work. Where a worktable might have a machinist’s vise bolted to its top, a woodworker’s bench is built to accommodate a number of different workholding mechanisms, such as bench dogs, planing stops, hold fasts, or board jacks, and will usually have one more woodworker’s vises integrated into its structure.
A workbench needs to be heavy enough that it doesn’t move under you while you’re working, and stiff enough that it doesn’t rack itself to pieces under the forces that will be placed upon it.
Watch the following video for a more in-depth discussion.