What is the best data structure for web classification, where each website can have multiple class labels?

I want to store a large number of websites, where each website can have multiple class labels. Ideally, I'd like to have a set of primary labels (entertainment, news, sports, etc.) for each website, and each of those primary labels can have a number of sub-classes (sports -> news and media -> ...). I want to be able to quickly query by class, as well by website. What is the best data-structure to accomplish this?