Because Adar 13 (the day before Purim) falls on a Shabbat this year, the "Fast of Esther," usually observed on that date, is moved back to today.
The fast -- which is observed today by all adults (i.e., over bar or bat mitzvah age) -- is in commemoration of the three-day fast called at Esther's behest before she risked her life to appear unsummoned before King Achashveirosh to save the Jewish people from Haman's evil decree (as related in the Book of Esther, chapter 4). The fast also commemorates Esther's fasting on the 13th of Adar, as the Jews fought their enemies (see "Today in Jewish History" for Adar 13). No food or drink is partaken of from daybreak to nightfall. Pregnant of nursing woman or people in ill health are exempted from fasting.