Drill-Down reports

what are Drill-Down reports?

This is a really good explanation and how I learned it:

Drill through reports:
A drill through report allows you to go from summary to detail. For instance: You have a financial application in which you can show the balance and profit and loss for a specific time frame. Every general ledger displays the total amount of all transactions for this specific period. By clicking (Drill through) on one of the general ledgers you will see the underlying transactions for this specific period.

Drill down reports:
A drill down reports allows you to look to the data in different levels. For instance, a report displays information on Country level if we drill down the report displays information on state level. Next drill down will display information on city level. So Countries contains States, States contains Cities. So we can view all levels through drill down.
Another examples is Year contains Months, Months contains Weeks, Weeks contains Days, Days contains Hours, Hours contains Minutes.