Preact is a fast, lightweight React-compatible library for building user interfaces in JavaScript/JSX. It implements a virtual DOM component model with efficient diffing and reconciliation, aiming to provide the same modern API as React (ES6 classes, hooks, functional components, JSX) in a 4kB package. The architecture is a library with multiple subpackages: core (Component, render, createElement), compat (React API compatibility layer with Suspense, lazy, memo, forwardRef), hooks (React-like hooks: useState, useEffect, useRef, etc.), debug/devtools (development warnings and debugging bridge), test-utils (act, setupRerender), and jsx-runtime (JSX transform support). Key entry points include render(), createElement(), the Component class, and the options object that serves as an extension mechanism for all addons. Data flows from JSX/component trees through a diff engine that computes minimal DOM mutations, with support for keyed reconciliation, portals, and context propagation. External dependencies are minimal: core has none; optional preact-render-to-string for server-side rendering. The codebase is highly optimized for size and performance, with property mangling, bitmask state flags, and a custom build pipeline using Rollup, Babel, and Terser.