A single family office serves one household and is typically set up above a threshold where dedicated staff become cheaper than external advice. A multi-family office serves several and looks more like a boutique advisory firm.
Because there is no external client base, the office can hold genuinely long-horizon and illiquid assets, take concentrated positions in an operating business, and skip the reporting cycles that constrain institutional managers. Governance, succession and conflict management are the recurring hard problems rather than portfolio construction.
Family offices are usually exempt from registering as investment advisers when they serve only family clients, which means lighter oversight and more responsibility on internal controls. See illiquidity-premium.
Related: illiquidity-premium, private-equity, fund-of-funds, concentration-risk, investment-policy-statement, accredited-investor