Al HaNissim is an addition to the Amidah and Birkat Hamazon on Hanukkah and Purim. On both holidays, it starts off with a short paragraph, beginning with the words for which it is named. After that, each holiday has a unique paragraph, describing the events for which that day is celebrated.
On Hanukkah, the prayer is recited to thank God for the miracles with which the holiday is associated. On Purim, it is recited because there is an obligation to recite the events of the day in one's prayers, but not the miracles of the day.