Default trait for sensible zero values

The Default trait provides a default value for a type, useful for builder patterns, config merging, and initialization. I derive Default on structs where zero/empty is meaningful. For custom logic, implement it manually. Default::default() is the standard way to create an instance. It's especially useful with struct update syntax: Config { port: 8080, ..Default::default() }. For types like Vec, String, and Option, default is already implemented (empty vec, empty string, None). I use Default bounds in generic code when I need to create instances. It's a small convenience that makes APIs more ergonomic and composable.