CTRs (Currency Transaction Reports) are required filings to the Financial Crimes Enforcement Network (FinCEN) to report all transactions and deposits in cash (in any currency) worth over $10,000.
This includes multiple transactions that add up to over $10,000. This rule is closely tied to Anti-Money Laundering (AML) rules and reporting requirements which have become more stringent since the turn of the century.
Banks are required to submit reports for all transactions over $10,000 in cash, whether it is a deposit, withdrawal, or other transaction. Most banks have systems to automatically submit these reports, which will also include any relevant information about the customer that they have on file.
These reports do not necessarily imply that anything suspicious has taken place, but if an employee of a bank or other institution believes that this is the case, they can also file a Suspicious Activity Report (SAR). These rules all stem from the Bank Secrecy Act of 1970, which was aimed at curtailing money laundering from domestic and foreign entities.