The fast of the 17th of Tammuz is observed this year on this coming Sunday (though the date is actually Saturday, we push it off so that we do not fast on the Sabbath).
This begins a 3 weeks period of mourning and introspection culminating in the fast of the 9th of the month of Av (also, pushed to a Sunday).
The first fast is a "dawn til nightfall" fast on which only eating and drinking are forbidden. The 9th of Av is a 25 hour fast which has additional limitations on behavior.
This is a reversal of the month of mourning after the loss of a family member. In that time of personal mourning, our sadness lessens slightly over a month so the restrictions get less and less. In this communal mourning (over the lass of the temples in Jerusalem and a host of other tragedies) the limitations increase.