React is a JavaScript library for building user interfaces, particularly single-page applications. It's component-based, efficient (virtual DOM), and has a strong ecosystem.

Key concepts include components (functional/class), JSX syntax, props and state, lifecycle methods (or hooks), virtual DOM, and unidirectional data flow.

React is a library focused on UI with flexibility in other choices, while Angular is a full framework with more built-in solutions but steeper learning curve.

Hooks (introduced in React 16.8) allow using state and other React features in functional components, simplifying code and reducing class component usage.

React creates a lightweight copy of the real DOM in memory. When changes occur, it compares (diffs) virtual DOM versions and updates only the changed parts in the real DOM.

JSX is a syntax extension that allows writing HTML-like code in JavaScript. It gets transpiled to React.createElement() calls that create React elements.

For local component state, use useState hook. For global state, consider Context API, Redux, MobX, or newer solutions like Recoil or Zustand.

Common tools include Create React App (starter), Next.js (SSR), React Router (navigation), Redux (state), and testing libraries like Jest/React Testing Library.

Use React.memo, useCallback, useMemo, code splitting, lazy loading, virtualized lists for large data, and avoid unnecessary re-renders.

Fragments let you group multiple children without adding extra nodes to the DOM, useful when a component needs to return multiple adjacent elements.