A C4 diagram describes the architecture of a software system at four levels of detail: Context, Containers, Components, and Code. The Context level — shown here — is the most accessible: it shows the system as a single box surrounded by the people and external systems it interacts with.

C4 diagrams are used in technical documentation, but the Context level is readable by anyone and is useful for communicating what a system does and who uses it, without getting into implementation details.

C4Context
    title Blog Platform — System Context

    Person(author, "Author", "Writes and publishes posts")
    Person(reader, "Reader", "Browses and reads articles")
    Person(subscriber, "Subscriber", "Receives the weekly newsletter")

    System(cms, "CMS", "Content management system for creating, editing, and publishing posts")

    System_Ext(email, "Email Service", "Sends newsletters and account notifications")
    System_Ext(analytics, "Analytics", "Tracks page views and reader engagement")
    System_Ext(search, "Search Engine", "Indexes published content for web search")

    Rel(author, cms, "Creates and manages content")
    Rel(reader, cms, "Reads posts")
    Rel(subscriber, cms, "Signs up for newsletter")
    Rel(cms, email, "Sends newsletters and alerts")
    Rel(cms, analytics, "Reports activity")
    Rel(search, cms, "Crawls published pages")

The C4 Context diagram is often the best starting point for explaining a system to a non-technical audience — it answers 'who uses this and what does it connect to?' without requiring any technical background.