Is 'Just Sitting' the same as zazen?
The term Zazen (Chinese 坐禪, zuò chán) just means seated meditation. It doesn't itself specify what kind of meditation is being practiced.

Just Sitting refers to Shikantaza, which is the type of meditation taught by Dōgen Zenji, the founder of the Sōtō school of Japanese Zen. He famously explained it in the Fukanzazengi saying :


  Once you have adjusted your posture, take a breath and exhale fully,
  rock your body right and left, and settle into steady, immovable
  sitting. Think of not thinking. Not thinking-what kind of thinking is
  that? Nonthinking. This is the essential art of zazen.


There are similar practices in other schools of Zen in China and Korea, but it is less prominent as Koan practice is more common.