1. A method comprising: receiving one or more seed sites that present information relating to a category of products or services, an individual seed site including a plurality of labeled text nodes identifying attributes targeted in association with data extraction for the category of products or services; extracting features from the plurality of labeled text nodes; learning, by a processor, category knowledge for the category of products or services based at least in part on the features extracted from the plurality of labeled text nodes, the category knowledge characterizing attribute-specific semantics of the plurality of labeled text nodes; receiving a new web site that presents information relating to the category of products or services; identifying, based on the category knowledge, one or more attributes on the new web site; and extracting one or more values from the one or more attributes identified.