What is the difference between interrogatories and admissions?

Both are based on information that is discoverable before trial as put forth in Federal Rule of Civil Procedure 26(b). Both are relatively inexpensive means of gathering information that can be used for trial preparation (although not all information will necessarily be admissible in a trial).

Interrogatories are described under Rule 33. They allow a party to ask the opposing party questions to clarify matters of fact or applications of law to fact. Parties may argue over the fundamental importance and application of matters from interrogatories and may be contradicted by other pieces of evidence. They are used primarily to identify potentially relevant witness and locations of information (or limited specific information) that was not a part of original disclosure. These often lead to further requests for specific documents or allow the requesting party to more carefully focus attention on relevant issues.

Requests for Admission, Rule 36, relate to the same type of issues as interrogatories, but are more like pleadings; an admission takes a matter entirely out of controversy. Admissions can be used as an efficient way to address ambiguous answers from an opposing party. Problems may arise if the admission in question is at the fundamental core of the dispute ("did you maintain your sidewalk negligently?") Once answered, if agreed upon, admissions are not disputed by other evidence.

Examples:

Interrogatory:

"Were there any employee statements taken at the time of the event or in the three weeks immediately subsequent to the event?"

"If so, include exact copies of said statements if in writing, and set forth in detail any such oral statements or reports."

Admission:

"Where is your principal place of business?"

"When was the sidewalk originally installed?"