Generally, when you buy an item out of the state of California (or over the internet with no California sales tax being paid) that is used, stored, consumed, or given away in California you owe use tax. In some cases, retailers that are engaged in business in California that sell to out of state purchasers must register with the BOE, collect use tax from their out of state customers, and pay it to the California Board of Equalization.
Consumers, other than retailers and qualified purchasers, may file use tax returns with the BOE or, alternatively in certain circumstances, they may report their use taxes on their California income tax return.