What are the duties of a federal district court regarding a report and recommendation made by a federal magistrate?

Federal Rule of Civil Procedure Rule 72(b) and 28 U.S.C. 636(b)(1) set forth a district court's duties regarding the magistrate judge's report and recommendation. The district court "shall make a de novo determination of those portions of the report . . . to which objection is made," and "may, accept, reject, or modify, in whole or in part, the findings or recommendations made by the magistrate judge." 28 U.S.C. 636(b)(1)(c); see also United States v. Raddatz, 447 U.S. 667, 673-76 (1980).