1. A computer-implemented method, comprising: collecting user activity information for a plurality of users, wherein the user activity information that has been collected for the plurality of users includes at least one of web pages that have been viewed by the plurality of users, content of the web pages that have been viewed by the plurality of users, advertisements that have been clicked on by the plurality of users, or user search results that have been selected by the plurality of users; generating by a processor a mapping model, wherein generating a mapping model includes mapping each one of a plurality of different user characteristics to a corresponding set of terms based, at least in part, on the user activity information that has been collected for the plurality of users and user profiles of the plurality of users, wherein each set of terms include advertisement terms that are a subset of an advertisement term list, the advertisement term list being separate from the user profiles, wherein the plurality of different user characteristics include a plurality of different categories that represent user interest or expertise in such different categories and/or a plurality of different user demographics; receiving a request for an advertisement to be displayed in a web page that has been requested by a user, wherein the user is associated with one or more user characteristics from the plurality of different user characteristics, the one or more user characteristics including one of the plurality of categories and/or one of the plurality of different user demographics, the one or more user characteristics being indicated by one of the user profiles; using the mapping model, for each one of the one or more user characteristics of the user, obtaining the corresponding set of terms; and providing the obtained terms for selecting one of a plurality of advertisements for displaying via the web page, wherein the advertisement term list is separate from the plurality of advertisements.
1. A computer-implemented method, comprising: collecting user activity information for a plurality of users, wherein the user activity information that has been collected for the plurality of users includes at least one of web pages that have been viewed by the plurality of users, content of the web pages that have been viewed by the plurality of users, advertisements that have been clicked on by the plurality of users, or user search results that have been selected by the plurality of users; generating by a processor a mapping model, wherein generating a mapping model includes mapping each one of a plurality of different user characteristics to a corresponding set of terms based, at least in part, on the user activity information that has been collected for the plurality of users and user profiles of the plurality of users, wherein each set of terms include advertisement terms that are a subset of an advertisement term list, the advertisement term list being separate from the user profiles, wherein the plurality of different user characteristics include a plurality of different categories that represent user interest or expertise in such different categories and/or a plurality of different user demographics; receiving a request for an advertisement to be displayed in a web page that has been requested by a user, wherein the user is associated with one or more user characteristics from the plurality of different user characteristics, the one or more user characteristics including one of the plurality of categories and/or one of the plurality of different user demographics, the one or more user characteristics being indicated by one of the user profiles; using the mapping model, for each one of the one or more user characteristics of the user, obtaining the corresponding set of terms; and providing the obtained terms for selecting one of a plurality of advertisements for displaying via the web page, wherein the advertisement term list is separate from the plurality of advertisements. 7. The method of claim 1 , wherein the mapping comprises maximizing a likelihood function.