You might come across these terms when looking for plywood: structural/external, non-structural/interior and marine-grade plywood. The difference lies in the glue (adhesive) used to manufacture the plywood and the plywood's strength, moisture-resistance and surface defects. Of course, these features determine where you can use the plywood and what you can use it for.
Simply put, marine-grade plywood is the strongest and most durable because it can withstand wet environments. It is followed by structural plywood and then non-structural plywood.