TL;DR Examining the core concept of separation of concerns.

Separation of concerns is about creating modular systems in which each module specialises and provides well focussed, self-contained functionality. Systems that are divided into highly cohesive, weakly coupled modules with coherent abstractions will generally also have good separation of concerns.

Examples

HTML, CSS, and JavaScript

A commonly encountered set of technologies are HTML, CSS, and JavaScript. HTML describes the structure of a document (web page), CSS describes how the document should be presented, and JavaScript provides the document’s behaviour. Each technology supports a specific concern1.

1At least that’s the intention. In practice there is considerable overlap as JavaScript can manipulate both presentation and structure, albeit via the other two.