The text of the mourner's Kaddish is is the same as the text of the regular Kaddish used throughout the service. It's normally a transition into and out of the major prayers, and usually it is sung/chanted. The only difference when it is the mourner's Kaddish is that it is said instead of sung. The explanation I've heard for this is that even/especially when confronting death it is important to remind oneself of the holiness of God. Also, that death isn't something separate or different, but part of God's holiness like everything else. I have no idea if that is an "official" explanation, but that's what I was taught.