7. An apparatus for automatically adding a tag to a document, comprising: a candidate tag word determining module comprising a processor, configured to determine a plurality of candidate tag words corresponding to the document; a co-occurrence probability determining module comprising a processor, configured to determine a corpus comprising a plurality of texts, select commonly-used words from the corpus as characteristic words, and determine, for each of the characteristic words and each of the candidate tag words, a probability for co-occurrence of the candidate tag word with the characteristic word; a weight calculating module comprising a processor, configured to abstract characteristic words from the document, and calculate a weight for each of the abstracted characteristic words; a weighted co-occurrence probability calculating module comprising a processor, configured to calculate, in the corpus, a weighted probability for co-occurrence of each of the candidate tag words with all of the characteristic words abstracted from the document; and a tag word adding module comprising a processor, configured to select the candidate tag word with a high weighted co-occurrence probability as a tag word to be added to the document; wherein the weight for the characteristic word Y abstracted from the document is denoted as W y , and the weight calculating module is configured to calculate W y as being equal to a product of the number of times that Y occurs in the document and the number of the texts in the corpus in which Y occurs.
7. An apparatus for automatically adding a tag to a document, comprising: a candidate tag word determining module comprising a processor, configured to determine a plurality of candidate tag words corresponding to the document; a co-occurrence probability determining module comprising a processor, configured to determine a corpus comprising a plurality of texts, select commonly-used words from the corpus as characteristic words, and determine, for each of the characteristic words and each of the candidate tag words, a probability for co-occurrence of the candidate tag word with the characteristic word; a weight calculating module comprising a processor, configured to abstract characteristic words from the document, and calculate a weight for each of the abstracted characteristic words; a weighted co-occurrence probability calculating module comprising a processor, configured to calculate, in the corpus, a weighted probability for co-occurrence of each of the candidate tag words with all of the characteristic words abstracted from the document; and a tag word adding module comprising a processor, configured to select the candidate tag word with a high weighted co-occurrence probability as a tag word to be added to the document; wherein the weight for the characteristic word Y abstracted from the document is denoted as W y , and the weight calculating module is configured to calculate W y as being equal to a product of the number of times that Y occurs in the document and the number of the texts in the corpus in which Y occurs. 16. The apparatus according to claim 7 , wherein the weighted co-occurrence probability calculating module is configured to calculate, in the corpus, the weighted probability for each of the candidate tag words that co-occur with more than one characteristic word abstracted from the document.