According to Forbes.com:
Upward of 180,000 California businesses are being ordered to register with the state's tax collection agency, the Board of Equalization, and to file a new tax return covering just one levy--the state's "use" tax, a sales tax on items purchased from out-of-state retailers--over the Internet, by telephone or by mail order--and then used in-state. The Supreme Court has ruled that only those out-of-state retailers with some physical connection to a state must collect its sales/use tax. If a retailer doesn't collect it, the buyer is supposed to voluntarily fork over the use tax to his own state.