1. A method for defining a content relevance model for a particular category, the content relevance model for determining whether a content segment is relevant to the particular category, the method comprising: receiving a first set of content segments that contain content previously determined to be relevant to the particular category and a second set of content segments that contain content previously determined to be not relevant to the particular category; identifying a set of key word sets that appear more frequently in the first set of content segments than the second set of content segments; and defining a content relevance model that comprises a set of groups of word sets and a score for each group, each of the groups of word sets comprising a key word set from the identified set of key word sets and at least one word set found in a context of the key word set in at least one of the received content segments, the content relevance model for scoring new content segments that are different from the content segments of the first and second sets of content segments, in order to determine relevance of the new content segments to the particular category.