The difference between the include element and the import element is that the import element allows references to schema components from schema documents with different target namespaces and the include element adds the schema components from other schema documents that have the same target namespace (or no specified target namespace) to the containing schema. In short, the import element allows you to use schema components from any schema; the include element allows you to add all the components of an included schema to the containing schema.
1. IBM Web Services Programming Tips
2. Microsoft notes on import
3. XML Schema Spec Part 1