Container: Multi-purpose widget for decoration, sizing, and positioning

Container(
  width: 100,
  height: 100,
  color: Colors.blue,
  padding: EdgeInsets.all(8),
  child: Text('Hello'),
)

SizedBox: Lightweight widget specifically for sizing and spacing

SizedBox(
  width: 100,
  height: 100,
  child: Text('Hello'),
)

Use SizedBox for simple sizing; Container when you need decoration or complex styling.