A ledger is a principal book of accounts that contains a summary of all financial transactions of a business. It is used to record and classify transactions according to accounts, such as assets, liabilities, equity, revenue, and expenses. The ledger provides a complete record of each account's balance, which can be used to prepare financial statements, such as the balance sheet, income statement, and cash flow statement. Overall, the ledger serves as the central repository of a company's financial information, providing an organized and comprehensive view of its financial position and performance.