What is the difference between a schematic and a module?

A schematic creates a finished item. It could be an EBK, weapon, cargo expander, settlement building…but it is a usable item somewhere in the game. The schematic will require resources and usually modules.

A module is a component or sub-component. The more advanced builds may have a build tree four or five components deep and can require very many different modules.

See this Crafting Guide for more information