DISTINCT checks the results of query and eliminates duplicate rows. These queries (Unique Values = Yes) are not updateable. They are a snapshot of your data and don't reflect subsequent data modifications by users. This is similar to running a Totals Query (e.g. using a Group By clause).
DISTINCTROW checks all the fields in the table and then eliminates the duplicate rows. The results of a query with DISTINCTROW (Unique Records = Yes) are updateable and reflect changes to retrieved records (but the query does not automatically run again if the data changes to retrieve different rows).
So the difference is that DISTINCT only checks the fields in the results, while DISTINCTROW checks all the fields in the underlying tables. If your query joins several tables and only displays records from one, the DISTINCTROW option lets you view and edit the results.