A shofar is a horn, traditionally that of a ram, used for Jewish religious purposes. During Elul, the month of preparation before the Jewish High Holidays, Jews are called to teshuvah (repentance/reflection/return) before the new year, and the shofar is a daily wake-up call.
The shofar is primarily blown on Rosh Hashanah and Yom Kippur, but it is also used during Sukkot, the Feast of the Tabernacles. The shofar is blown in synagogues to mark the end of the fast at Yom Kippur and blown at four particular occasions in the prayers on Rosh Hashanah. Three main patterns of sound -- tekiah, teruah and sh'varim -- are used when blowing the shofar in a synagogue, and full cycle of sounds numbers 100 blasts.