Explain Domain-Driven Design (DDD) concepts and when to apply them.

DDD focuses on complex domains, aligning code with business. Strategic patterns: Bounded Context (explicit boundaries around models), Ubiquitous Language (shared vocabulary), Context Mapping (relationships between contexts). Tactical patterns: Entities (identity), Value Objects (immutable, equality by value), Aggregates (consistency boundary), Domain Events, Repositories, Domain Services. Apply to complex domains with significant business logic. Overkill for CRUD apps. Requires domain expert collaboration. Often combined with hexagonal architecture.