The four minor fasts on the Jewish calendar commemorate tragedies in Jewish history such as the murder of Gedaliah, the Jewish governor of Judah, and the breach of the walls around Jerusalem in 70 CE. Unlike major fast days, which last 25 hours, these minor fasts last only during daylight hours.
In addition to the major public fast days of Yom Kippur (the Day of Atonement) and Tisha B’Av (the Ninth of the month of Av, which commemorates the destruction of the Temples ), there are four minor public fast days in the Jewish calendar. The minor fasts last only from dawn until sundown, in distinction to Yom Kippur and Tisha B’Av, which last 25 hours, from sundown until three stars can be seen in the sky the following night. The minor fast days also are not as restrictive in practice as the major fasts.
With the exception of Yom Kippur, none of the other communal fast days in the Jewish calendar may fall on the Sabbath. Hence, the Jewish calendar must sometimes be adjusted in order to ensure that these fasts are observed on other days of the week. On the minor fasts, only eating and drinking are restricted, not other signs of mourning, such as wearing leather, that are prohibited on major fast days. In addition, the laws concerning who may break his or her fast–or skip the fast altogether–are more lenient for minor fasts.
In addition to various additional readings in the synagogue on these days, one of the characteristic traditions associated with these fast days is the giving of charity or tzedakah .
Material excerpted from http://www.myjewishlearning.com/.