29. One or more computer-readable storage media embodying software operable when executed by one or more computer systems to: access a plurality of keywords, each of the keywords comprising one or more words and describing one or more characteristics or features of particular advertising subject matter; annotate each of the keywords with one or more labels selected from a plurality of labels, each of the labels comprising one or more words and describing one or more aspects of or one or more categories or concepts represented by the keyword, wherein annotate each of the keywords with one or more labels comprises: for each of the keywords, when the keyword is able to be directly annotated, then directly annotating the keyword; and when the keyword is unable to be directly annotated, then: construct a keyword document for the keyword; for each of the labels, construct a label document for the label; and calculate a similarity between the keyword document corresponding to the keyword and the label document corresponding to the label as ∑ k = 1 k = n w ⁢ ⁢ wc i , k ⁢ wc j , k  wc i  ⁢  wc j  , where: (1) n W denotes a total number of unique words in the keyword document and the label document; (2) wc i,k denotes a number of times a specific word, word k , occurs in the keyword document; (3) wc j,k denotes a number of times the specific word, word k , occurs in the label document; (4) wc i denotes a n W -vector consisting of the wc i,k ; and (5) wc j denotes a n W -vector consisting of the wc j,k ; and annotate the keyword based on the similarity between the keyword document corresponding to the keyword and the label document corresponding to each of the labels; group the keywords into one or more keyword groups based on similarities between the labels of the keywords, each of the keyword groups comprising one or more of the keywords; and form one or more advertising groups from the keyword groups, comprising: from each of the keyword groups, form one or more of the advertising groups, each of the advertising groups comprising one or more of the keywords in the keyword group.
29. One or more computer-readable storage media embodying software operable when executed by one or more computer systems to: access a plurality of keywords, each of the keywords comprising one or more words and describing one or more characteristics or features of particular advertising subject matter; annotate each of the keywords with one or more labels selected from a plurality of labels, each of the labels comprising one or more words and describing one or more aspects of or one or more categories or concepts represented by the keyword, wherein annotate each of the keywords with one or more labels comprises: for each of the keywords, when the keyword is able to be directly annotated, then directly annotating the keyword; and when the keyword is unable to be directly annotated, then: construct a keyword document for the keyword; for each of the labels, construct a label document for the label; and calculate a similarity between the keyword document corresponding to the keyword and the label document corresponding to the label as ∑ k = 1 k = n w ⁢ ⁢ wc i , k ⁢ wc j , k  wc i  ⁢  wc j  , where: (1) n W denotes a total number of unique words in the keyword document and the label document; (2) wc i,k denotes a number of times a specific word, word k , occurs in the keyword document; (3) wc j,k denotes a number of times the specific word, word k , occurs in the label document; (4) wc i denotes a n W -vector consisting of the wc i,k ; and (5) wc j denotes a n W -vector consisting of the wc j,k ; and annotate the keyword based on the similarity between the keyword document corresponding to the keyword and the label document corresponding to each of the labels; group the keywords into one or more keyword groups based on similarities between the labels of the keywords, each of the keyword groups comprising one or more of the keywords; and form one or more advertising groups from the keyword groups, comprising: from each of the keyword groups, form one or more of the advertising groups, each of the advertising groups comprising one or more of the keywords in the keyword group. 36. The media of claim 29 , wherein the software being further operable when executed by the one or more computer systems to: construct a classifier based on a plurality of training keywords and a plurality of training keyword documents, each of the training keywords comprising one or more words and uniquely corresponding to one of the training keyword documents, comprising: for each of the training keywords, annotate the training keyword with one or more of the labels based on the training keyword document corresponding to the training keyword; and for each of the labels annotating the training keyword, determine a correctness of the label; calculate a first index-wise product between a word count vector of the training keyword document corresponding to the training keyword and a word count vector of the label document corresponding to the label; and form a pair of the correctness and the first index-wise product; and train the classifier using the pairs of the correctness and the first index-wise product; and for each of one or more of the keywords, for each of the labels annotating the keyword, calculate a second index-wise product between a word count vector of the keyword document corresponding to the keyword and a word count vector of the label document corresponding to the label; and predict whether the label annotating the keyword is correct using the classifier with the second index-wise product as an input to the classifier.