What is ancestor worship?

Answer 1

Ancestor worship refers to the worship of one ancestors as the term suggests. It was, and perhaps still is believed that ones deceased ancestors will act on your behalf in communing with God.

Let's Think Further (In the Case of China)

The term "ancestor worship" is perhaps misleading. If we go back to the oracle bones from the Shang dynasty (1600-1046 BCE), the idea is best understood as "entertaining one's ancestor" (bin). The idea is rather like a socialising process. A community hosted or entertained the dead and honoured the dead as "ancestors." The belief was that once these "ancestors" were satisfied, they would in turn entertain their ancestors. The system of imagined entertainment between hosts and guests worked like a pyramid, until the "eldest ancestors" (and in terms of honour, the highest) were entrusted with the job of entertaining the "high god" (take a look at Michael Puett's To Become a God).

This belief system and its variations survived throughout the imperial period (arguably, up until 1911) as the dominant thinking. However, as you can see, such system can be boiled down to a wish to please the "high god" in order to guarantee material gains (to win a war, to have favourable weather, etc.). In response to such materialistic belief, schools of philosophy that question its lack of ethical values and transcendental dimensions emerged at different historical moments (e.g. what we now call Daoism, Confucianism, etc.; again please refer to Puett's analysis).

Of course, on a larger political scale (ref. the "Fengchan shu" chapter of the Shiji), or in fact, as private practices, what we now regard as "ancestor worship" are often pro forma rituals that renew the sense of in-group solidarity among the living, or, in some cases, a sense of self-satisfaction or fulfilment.

Answer 2

Ancestral worship is just what the name says : worship of their ancestors. such is found in Native American "worship", and has strong ties in Buddhism and other asian "worship"