[A human Being]

Originally Posted by JustASimpleGuy

As I understand Zazen is usually associated with study of koans whereas Shikantaza is the version devoid of study of/contemplation on koans. I believe it's also called just sitting and I know it was resting in awareness, choiceless awareness and do nothing meditation.

But yeah, for me Shikantaza was the breakthrough practice.

I'd always equated Zazen with Shikantaza, though apparently it varies from school to school - from Wikipedia (link here):

In the Japanese Rinzai school, zazen is usually associated with the study of koans. The Sōtō School of Japan, on the other hand, only rarely incorporates koans into zazen, preferring an approach where the mind has no object at all, known as shikantaza.

According to that link, zazen practice is generally taught in one of three ways, the third way being concentration. I personally prefer just sitting (contemplating koans usually just leads to frustration for me, don't think I'm hooked up for it - I always sucked at riddles, too), though I like that there are different approaches, because we're not all wired up the same and therefore what works for one person may not work for another.