A government shutdown occurs when a government discontinues providing services that are not considered "essential." Typically, services that continue in spite of a shutdown include police, fire fighting, armed forces, utilities and corrections.
A shutdown can occur when a legislative body (including the legislative power of veto by the executive) cannot agree on a budget financing its government programs for a pending fiscal year. In the absence of appropriated funds, the government discontinues providing non-essential services at the beginning of the affected fiscal year. Government employees who provide essential services, often referred to as "essential employees", are required to continue working.