A control account is a summary account in the general ledger. The details that support the balance in the summary account are contained in a subsidiary ledger—a ledger outside of the general ledger.
The purpose of the control account is to keep the general ledger free of details, yet have the correct balance for the financial statements. For example, the Accounts Receivable account in the general ledger could be a control account. If it were a control account, the company would merely update the account with a few amounts, such as total collections for the day, total sales on account for the day, total returns and allowances for the day, etc.
The details on each customer and each transaction would not be recorded in the Accounts Receivable control account in the general ledger. Rather, these details of the accounts receivable activity will be in the Accounts Receivable Subsidiary Ledger. This works well because the employees working with the general ledger probably do not need to see the details for every sale or every collection transaction. However, the sales manager and the credit manager will need to know detailed information on individual customers, including whether a customer recently reduced their account balance. The company can provide these individuals with access to the Accounts Receivable Subsidiary Ledger and can keep the general ledger free of a tremendous amount of detail.