What is the difference between Lookup, LookupRows, and LookupOrderedRows in AMPscript?

Lookup returns a single scalar value — the value of the specified column from the first row matching the filter criteria. Syntax: Lookup("DE_Name","ReturnCol","FilterCol","FilterVal"). LookupRows returns a rowset object containing all matching rows. You iterate using Row(rowset, rowNum) and Field(row, "colName"). LookupOrderedRows returns a rowset sorted by a specified field (ascending or descending) and can be capped to a maximum row count. Use LookupOrderedRows when you need the most recent N records (e.g., last 3 purchases ordered by date descending).