To make the structure of a web application works a little clearer, a typical technology stack is illustrated here.
Data: the core data of the system will be stored in a database comprising a number of tables. Data can be validated at this level, but business rules are not implemented at this layer. This can be thought of as the ‘warehouse’ or ‘memory’ part of the solution
Business Logic Layer: this part of the stack reflects the requirements in the functional and technical specification to ensure the solution functions as required. This can be thought of as the ‘brain’ part of the solution
UI Layer: the UI (User Interface) is what the user ‘sees’ and interacts with when accessing the solution
Presentation Layer Business Logic: this part of the UI governs how the data is presented to the user. It is not as complex as the business logic layer, but can do some of the simple ‘thinking’ without communicating with the ‘brain’. The UI can be thought of as the ‘shop window’ or ‘face’ of the solution
Back-end: the data and business logic layers are often called the ‘back-end’ – users do not directly access or see these parts of a system
Front-end: the UI is often called the ‘front-end’ – it is what the user sees of the system
See also: CMS Technology Stack