An eCommerce platform is software that online businesses use to build functions needed to conduct business. The platform may be as simple as a catalog and shopping cart, but traditionally “platform” refers to a more robust set of tools and services.
These tools and services would include the catalog, shopping cart, customer management, analytics, fulfillment, accounting (or integration with accounting), communications tools, and more.
There are many providers of eCommerce platforms including hosted/SaaS offerings like BigCommerce or Shopify, customizable platforms you can run on your own servers or cloud like Magento, or add-ons to ERP systems like SAP/Hybris or Netsuite (SaaS ERP and commerce).