The first time a visitor comes to your web site, Yahoo! Web Analytics sets a one-year, first-party, persistent cookie in their browser. During the first visit, Yahoo! Web Analytics identifies them as a first time visitor. Next time that person visits your site, and every subsequent time, Yahoo! Web Analytics resets the cookie life to one year beyond the date of the visit, and identifies them as a returning visitor for the duration of that visit. Third-party cookies are only in use for the duration of the visit session and are only used for cross-domain tracking.
If Yahoo! Web Analytics is unable to set a cookie, then it will identify the visitor as a first time visitor. If the visitor has deleted the cookie, then Yahoo! Web Analytics will identify them as a first time visitor and re-issue a new first-party cookie.