Kaddish Yatom (Mourner’s Kaddish) is a prayer in Aramaic in which the hope is expressed that God’s great name will be sanctified in the whole world He has created and the Kingdom of Heaven be established on earth.
Mourner’s Kaddish is recited at least once during each of the 3 weekday (Ma’ariv, Shacharit, Mincha) and 4 shabbat (Ma’ariv, Shacharit, Musaf, Mincha) services.
There are any number of customs relating to Mourner’s Kaddish and mourners should consult with the officiating clergy as to who says Mourner’s Kaddish and the amount of time after the funeral that the Mourner’s Kaddish is said.
As a general rule, Mouner’s Kaddish is recited for 11 months for a parent, and less time for other family members. There are many customs depending on the mourner’s community and clergy should be consulted with any questions.