A balance sheet is the financial statement that reports a business's assets, liabilities and owners' equity as of a single date, usually the last day of a month, quarter or year. The SEC's plain-language guide puts the logic in one line, "ASSETS = LIABILITIES + SHAREHOLDERS' EQUITY", and describes the statement as "a snapshot" that "does not show the flows into and out of the accounts during the period." Nonprofits call the same statement a statement of financial position; federal student-aid rules, for instance, ask a for-profit school for a balance sheet and a nonprofit one for a statement of financial position.
It is one of three statements a business owner, lender or buyer reads together. The profit and loss statement reports revenue and expenses over a period, the cash flow statement reports cash moving in and out over the same period, and the balance sheet reports where things stand at the end of it. A household's equivalent, everything a family owns and owes, is a personal balance sheet; it follows the same arithmetic but answers a different question.