In the Prayers of "Yizkor," "May He Remember," we ask Hashem to "remember" the souls of our loved ones who have passed away. These include primarily family members: husbands and wives, parents and grandparents, brothers and sisters, aunts and uncles, and children, G-d Forbid.
But of course, as far as remembering is concerned, we know that Hashem never forgets, as we say on Rosh HaShanah, in the Prayer of "Zichronot," Remembrances, "Atah Zocher Maaseh Olam," You remember everything that has ever happened." So Yizkor is really an opportunity for us to bring to mind intensely, to re-create, if only for a few brief moments, connections that once existed between ourselves and loved ones, who have passed away.
It is also an opportunity to secure additional merit for those beloved ones by performing the act of "Tzedakah," Charity. This would be in accordance with the verse, "U'Tzedakah Tatzil MiMavet," "Charity Protects from Death," in the sense that the act of kindness involved in one's assisting the poor accrues not only to one's own merit, but also to the Eternal merit of one's parents and loved ones, whose standing in "Olam HaBa," the World of the Spirit, is enhanced by the acts of goodness done by those who remember them.
This is especially significant in connection with the Yizkor of Yom Kippur, when the living and the dead are visited by the Judge of All Worlds, and Atonement is sought by the living for themselves as well as for departed generations.
We also say Yizkor for two general groups, whose members frequently include one's family members, as well, but which extend outward to the general Jewish Family. In this grouping are included Martyrs of the Holocaust perpetrated by the Nazis, may Hashem erase their name, against our People, together with Anti-Nazi Resistance Fighters who lost their lives in their desperate struggle. The other general group of heroes include members of para-military organizations such as Irgun and Lechi, who preceded the State of Israel, and members of the Israel Defense Forces, who died in battles in behalf of our People.
Yizkor is also recited on the three major Festivals, Pesach (Passover), Shavuot (The Feast of Weeks) and Sukkot, holidays on which the Jewish People are obligated to appear not empty-handed, at the Temple in Jerusalem. We fulfill the "non-empty-handed" part of the obligation by generous pledges to "Tzedakah," and by redeeming them after the holiday.
It is traditional to recite verses at the beginning of the Yizkor Service which are not, strictly speaking, part of the service, but which reflect the spirit of Yizkor. That is, our awareness of the fragility and mortality of human beings, and our complete dependence upon Hashem for purpose in our lives, redemption, and eternal life.
"E-l Malei Rachamim," "G-d Who Is full of Mercy"
This prayer is recited by the "chazan," or prayer leader, in behalf of all the deceased, for whom the Yizkor Service was said, for each grouping mentioned above.
"Av HaRachamim," "Merciful Father"
This Prayer has undergone an Evolution. Originally, it was recited only in the weeks preceding Shavuot and Tisha B'Av, because those times were associated with historical massacres and catastrophes which befell the Jewish People.
Now, on days when Tachanun (a daily memorial prayer) would not be said, for example Rosh Chodesh, Av HaRachamim is said by the entire congregation in behalf of all the martyred heroes of the Jewish People.
In our times, a transition is taking place. Almost until the present, everyone who had living parents would leave the Synagogue during the saying of all of Yizkor, including those aspects dedicated to the Martyrs of the Holocaust and the fallen heroes of the Israel Defense Forces, which really apply to all of the Jewish People.
Many synagogues currently have the Yizkor Services organized such that the entire congregation remains "inside" for those aspects of the Service which apply to all Jews, and only those who do not have both parents remain inside for the family-specific Yizkor.
Until the 1970's, approximately, there existed a phenomenon in American Jewry known as the "Yizkor Jew." This individual would come to the synagogue basically only when Yizkor was said, perhaps also on Yom Kippur, thus belonging to both the "Yizkor Jew" and the "Yom Kippur" Jew clubs. On Yom Kippur. he would sit in the rear of the synagogue "shmoozing," chatting, with his neighbors or snoozing. Mainly for his benefit, synagogues would estimate the exact time for Yizkor's appearance in the Service, and Services might be held up in case of early arrival at the Yizkor point, and woe to the Prayer-leader and the Rabbi if the time was exceeded.
Nowadays, Yizkor Jews are hard to find. This basically proves the following point: Judaism is a living body of law, custom and faith. There is no way that it can survive on sentiment alone, and the only way to maintain one's own link with past generations and certainly the only way to pass one's Jewish heritage to future generations is to make a full commitment to the Jewish way of life.