What is the difference between Shikantaza and Zazen? I used to think that Zazen was synonymous with just sitting but I'm starting to doubt that. Is it Shikantaza that is synonymous with just sitting practice? It would help me if someone could define them and bring out the relationship between the two and the relationship between them both and just sitting practice. Thank you.
Shikantaza is a kind of Zazen. Any Zen meditation is called Zazen, including breath meditation, koan meditation, observing-the-thoughts meditation, and just-sitting-meditation (shikantaza).
To quote my buddhist friend J. from Japan:
In shikantaza you don't focus on anything, just sit, until there is no observer, you really do nothing besides sitting there without reason, focus, observing, eventually you become thoughtless, and eventually unbecome you. No more than the wall, or better, everything and nothing.
Also see my answer about relationship between different kinds of Zazen.