There are different types of fasting, because fasting could be used to achieve spiritual results, sometimes fasting can be used for loosing weight, or fasting could also be used for medical purposes. 

The fasting of Tamuz & Tisha B’ Av
The two days of fasting which is done in within the 3 weeks of mourning on the Hebrew Jewish calendar.

On the 17th Tamuz there is fasting that is done, and the second fasting is done on the 9th Av Tisha B’ Av, being also the last day which closes the 3 weeks of mourning.

The fasting aims at the mourning of the destruction of the temple, and the loss which came as a result of the negative incidents that have occurred during the month of Tamuz and Av.

Just liken unto a person who loses apatite for food because of the loss or death of the one he loves, the person refrains from eating, and doing things that brings pleasure to one’s soul as a sign of lamenting (mourning) for the loss of his loved one.

So it is with the 2 days of fasting, and mourning that is done in the season of the 3 weeks.
The two days being the 17th of Tamuz, and the 9th of Tisha B’Av is a time of fasting, combined with the 3 of weeks mourning.

The bible or Torah studies done during the season:
Texts such as the Book of Lamentations, the Book of Job, portions of Jeremiah, the book of Revelation 2:1-Revelation 3:22 and chapters of the Talmud that discuss the laws of mourning.