1. A system for selecting an advertisement for display with a web page requested by a user through a web browser, the system comprising: at least one server, in communication with said web browser, configured to: receive, as input, a plurality of documents and a plurality of key terms; analyze content of the plurality of documents; automatically generate a key term feature vector for each key term based on the content of the plurality of documents, each key term feature vector comprising elements comprising a plurality of words or phrases that are related to the corresponding key term; analyze at least one of a Universal Resource Locator (“URL”) of said web page and a content of said web page; automatically generate a document feature vector based on the URL or the content of the web page, the document feature vector comprising elements comprising a plurality of words or phrases that are contained in the URL or content of the web page; compare the key term feature vector and the document feature vector; generate a relevance factor based on the similarity of the elements of the key term feature vector and the document feature vector; and provide a relevant advertisement for display by said web browser with said web page according to the determined relevance factor.