In software engineering, the terms front end and back end refer to the separation of concerns between the presentation layer (front end), and the data access layer (back end) of a piece of software, or the physical infrastructure or hardware. In the client–server model, the client is usually considered the front end and the server is usually considered the back end, even when some presentation work is actually done on the server itself.
In software architecture, there may be many layers between the hardware and end user. The front is an abstraction, simplifying the underlying component by providing a user-friendly interface, while the back usually handles data storage and business logic.
In telecommunication, the front can be considered a device or service, while the back is the infrastructure that supports provision of service.
A rule of thumb is that the client-side (or "front end") is any component manipulated by the user. The server-side (or "back end") code usually resides on the server, often far removed physically from the user.
Another way to understand the difference between the two is to understand the knowledge required of a front-end vs. a back-end software developer. The list below focuses on web development as an example.
Note that both positions, despite possibly working on one product, have a very distinct set of skills.
In network computing, front end can refer to any hardware that optimizes or protects network traffic. It is called application front-end hardware because it is placed on the network's outward-facing front end or boundary. Network traffic passes through the front-end hardware before entering the network.
In processor design, front end design would be the initial description of the behavior of a circuit in a hardware description language such as Verilog, while back end design would be the process of mapping that behavior to physical transistors on a die.