Query keys are unique identifiers that React Query uses for caching, invalidation, and refetching. They must be arrays and are compared deeply.
Basic structure:

// Simple key
queryKey: ['posts']
// Key with parameters
queryKey: ['posts', userId]
// Key with complex parameters
queryKey: ['posts', { userId, status: 'published' }]

Key principles:

• Uniqueness: Different keys create separate cache entries
• Hierarchy: Keys are hierarchical, allowing group operations
• Deep comparison: React Query compares keys deeply to determine if data should be refetched
  Examples:

// These are different queries with separate cache
queryKey: ['posts'] // All posts
queryKey: ['posts', 1] // Posts for user 1
queryKey: ['posts', 2] // Posts for user 2
// Invalidation example
queryClient.invalidateQueries({ queryKey: ['posts'] }); // Invalidates all post queries
queryClient.invalidateQueries({ queryKey: ['posts', 1] }); // Only user 1's posts

Query keys enable React Query's intelligent caching and make it easy to manage related data.