Regular assessments (dues) fund the annual budget. A special assessment covers something the budget and reserves can't, such as an unexpected roof replacement, storm damage or a legal judgment.
The board's power to levy one comes from the association's governing documents and state law. Many documents cap how large a special assessment the board can approve on its own and require an owner vote above that amount, so check yours before announcing one.
Special assessments are usually divided among units by the same formula as regular dues and can be due in one payment or spread over installments. Clear notice, the reason for the charge and a payment schedule go a long way toward owner acceptance.