In a pictorial view, you can see how the different models work together. Let’s look at this from the highest level; the External Model.
The External Model is the end user view of the data. Typically a database is an enterprise system that serves the needs of multiple departments. However, one department is not interested in seeing the other departments’ data. E.g. Human Resource does care to view the Sales data. Therefore, one user view will differ from another.
The External model requires that the designer subdivide a set of requirements and constraints into functional modules that can be examined within the framework of their external models (i.e. HR vs. Sales)
As a data designer you need to understand all the data so that you can build an enterprise wide database. Based on the needs of various departments the conceptual model is the first model created.
At this stage the conceptual model is independent of both software and hardware. It does not depend on the DBMS software used to implement the model. It does not depend on the hardware used in the implementation of the model. Changes in either hardware or DBMS software have no effect on the database design at the conceptual level.
Once a DBMS is selected, you can then implement it. This is the Internal model. Here you create all the tables, constraints, keys, rules, etc. This is often referred to as the Logical design.
The physical model is simply the way the data is stored on disk. Each database Vendor will have their own way of storing the data.