How is a "split system" different from a "package" unit?                               

A split system simply means that the equipment is "split" into two separate indoor and outdoor components.

If you have a furnace inside your home and an air conditioner outside in the yard, you have a split system.

A package unit means your heating and cooling system is self-contained in one outdoor unit. If your ductwork connects directly to your outdoor unit, then you have a package unit.