What is Domain-Driven Design and how does it apply to custom software?

Domain-Driven Design (DDD) is a software design approach that models software around the core business domain. Key concepts: Ubiquitous Language (shared vocabulary between developers and domain experts used in code and conversations), Bounded Contexts (explicit boundaries where a particular domain model applies), Aggregates (clusters of domain objects treated as a single unit for data changes), and Domain Events (significant occurrences in the business domain). Strategic patterns: Context Mapping identifies relationships between bounded contexts (shared kernel, customer-supplier, anticorruption layer), and the Big Picture Event Storming workshop discovers domain events, commands, and aggregates collaboratively. Tactical patterns: Entities (objects with identity), Value Objects (immutable objects without identity), Repositories (data access abstraction), Domain Services (operations that don't belong to a single entity), and Application Services (orchestrate use cases). Implementation: organize code by bounded context (not by technical layers), implement domain logic in the domain layer without framework dependencies, use the repository pattern to abstract persistence, and protect invariants within aggregates. DDD is most valuable for complex business domains with intricate rules and multiple stakeholders. For simple CRUD applications, DDD adds unnecessary complexity. Apply DDD selectively to the core domain (competitive advantage) and use simpler patterns for supporting and generic subdomains.