Understanding Modules

Modules are best understood as data storages. They keep basic data (Ecommerce Module keeps item categories, items, item fields as well as orders placed by your customers; Membership Module keeps it's member categories, members and member fields; etc.) and core settings (such as Shipping and Tax settings, Global Discounts and Coupons, etc.)

This way, modules represent your site back end your visitors never gain direct access to.