What is Apollo Client and how does it work with GraphQL?

Apollo Client is a comprehensive state management library for JavaScript applications that handles GraphQL queries, mutations, and subscriptions. It features an intelligent normalized in-memory cache that deduplicates data across queries — fetching a user in one query makes that data available for another query requesting the same user. Apollo Client handles request lifecycle (loading, error, data states), optimistic UI updates (update UI before server confirms), and refetching. It integrates with React via hooks (useQuery, useMutation), Vue, Angular, and more. Its developer tools extension lets you inspect the cache and active queries.