11. A method for recommending a keyword, the method comprising: determining whether the keyword is a direct click keyword by determining, referring to a click log, whether the keyword is associated with a first universal resource locator (URL) of an advertisement of an advertiser; determining whether the keyword is an indirect click keyword by determining that the keyword does not locate the first URL but locates a second URL having an attribute similar to the first URL; recommending the determined indirect click keyword; and computing selection probabilities that the first URL is selected using the direct click keyword and the indirect click keyword, wherein the recommending selects at least one indirect click keyword having a corresponding one of the selection probabilities greater than zero from among the plurality of indirect click keywords and then provides the selected at least one indirect click keyword to a user, and the computing includes, computing the selection probabilities based on selection rates of the first URL and the second URL with respect to the direct click keyword and the indirect click keyword, and a similarity probability between the first URL and the second URL, and updating the similarity probability based on the selection probabilities.
12. The method of claim 11 , wherein: the similarity probability is computed based on a rate of the first URL and the second URL selected by a plurality of searchers using the same keyword, and the rate is determined by referring to the click log.