Lookup fields create a loose relationship between two records. They are also built on the child. In contrast, users can edit lookup fields and search for a record to associate. Lookup fields also appear in the interface as a clickable field that navigates you to the associated record.

Rollup summary fields are build on the master (parent) in a Master-Detail (tight relationship). They can also be created on a select list of standard related objects (see link). Rollup summary fields calculate values from related child records.