What is an internal system?
An internal system is software built around a business's own operations, such as planning, quotes or stock, used only by the business itself.
Definition
An internal system is software built around a business's own processes, such as planning, quotes, stock or customer management, used only by the business itself rather than by visitors of the website.
Context
Where a website faces outward, an internal system is about what happens inside the business. It often replaces a collection of spreadsheets and separate tools that grew over the years: a planning in Excel, quotes in Word, customer data in the mailbox. While the business is small that works, until the moment nobody is sure anymore which version is the right one.
An internal system does not have to sit apart from the website. When both run on the same platform, an enquiry from the site flows straight into the planning, and a customer sees the same data in a portal as the business itself. That makes the difference between software as an extra burden and software that takes work off your hands.