Kaddish is special in that its content deals mainly with the respect and glory of Heaven (kevod Shamayim). Therefore, one must have great kavanah in his response, be careful not to let his mind wander, and not chatter while it is being recited (Shulchan Aruch 56:1; Mishnah Berurah 1). The Chachamim say about anyone who answers “Amen, Yehei Shemei rabba…” with complete concentration, that a harsh decree of seventy years of judgment against him is torn up (Shabbat 119b; Talmidei Rabbeinu Yonah). Further, they say that when Jews enter the synagogues and recite “Yehei Shemei rabba mevorach” out loud, their harsh decrees are nullified (Pesikta, as cited there by the Tosafot). Additionally, they state that the response to Kaddish arouses Hashem to grant mercy to Israel in exile. When Jews enter synagogues and study halls (batei midrashot) and respond, “Yehei Shemei HaGadol mevorach,” the memory of the exiled Jews ascends before HaKadosh Baruch Hu, Who shakes His head in pain, so to speak, and says, “Fortunate is the king who is glorified this way in his house,” awakening the desire to redeem Israel before Him (see Berachot 3a).
By reciting Kaddish, Hashem is sanctified. Therefore, it must be recited in a minyan, for Hashem is only sanctified by an “eidah” (a group) of Jewish people. Kaddish was composed in Aramaic, the language in which all Jews were fluent during the Second Temple period. The following is a translation of the prayer: “Exalted and sanctified be His great Name in the world which He created according to His will. And may He rule His Kingdom, and produce His salvation and draw His messiah near, in your lifetime and in your days (those of the congregation praying), and in the lifetime of the entire House of Israel, speedily and soon, and say, Amen.” The congregation responds, “Yehei Shemei rabba mevorach l’Alam ul’almei almaya,” which is translated as, “May His great Name be blessed forever and ever.” The chazan continues, “Blessed, praised, glorified, exalted, uplifted, honored, elevated, and lauded be the Name of the Holy One, Blessed is He, beyond all blessings, songs, praise, consolations uttered in the world, and say, Amen.”
That main part of the Kaddish is also called “Half-Kaddish.” Responding to it is of utmost importance, beyond compare. Even a person who is in the middle of Birkot Keriat Shema is permitted to stop to answer Amen. It is a mitzvah to run to hear Kaddish. One who already finished praying and has before him two minyanim, one reciting Kaddish and the other Kedushah, it is preferable that he join the minyan reciting Kaddish, for the virtue of Kaddish is greater than that of Kedushah (Mishnah Berurah 56:6).
. According to the opinion of the Mishnah Berurah 66:17, one who is in the middle of Birkot Keriat Shema or Pesukei d’Zimrah responds to the Kaddish in two places: “Amen, yehei Shemei rabba mevorach l’alam ul’almei almaya,” and also “Amen” at the end. According to the Kaf HaChaim 66:23, he responds with all five Amens in the main part of the Kaddish. See the laws of Birkot Keriat Shema in this book, 16:5.
It is important to state that according to Minhag Sephard, the chazan says “v’yitzmach purkanei vikarev meshichei” in the wording of the Kaddish and the congregation responds Amen, whereas in Minhag Ashkenaz, those four words are not recited. The root of this difference lies in the teachings of the Geonim. In Siddur Rav Amram Gaon and Machzor Vitri, the Kaddish is written like Minhag Ashkenaz, and in Siddur Rav Sa’adyah Gaon and in the Rambam it is like Minhag Sephard, although only “v’yitzmach purkanei” is mentioned (see Netiv Binah, part 1, p. 366). Further, it should be noted that in the main part of the Kaddish this is the only distinction; however, at the end of the Kaddish, there are many differences, for in the main part of the prayer the original nusach that Chazal instituted was better preserved. But, in the additions made by the sages of the generations after the scattering of the exiles, the discrepancies between the wordings are more pronounced.