Zazen is derived from the word chan (Chinese), which derives from dhyana (Sanskrit) or jhana (Pali), which basically translates to meditation. Zazen is actually more than just sitting in meditation, its continous practice whether standing, sitting, or lying down. Most people, however, commonly associate zazen as sitting in meditation in a zendo.
Zazen meditation is usually made up of 3 things. One can either do breathing meditation, focusing on a koan, or shikantza. I’m not really sure how to describe shikantaza other than it has some similarities to vipasanna. I’ve heard a zen teacher compare shikantaza to mahamudra. It’s similar in that both focus on awareness, but I don’t believe that comparison is entirely accurate. Mahamudra has its roots in tantra (i.e. requiring transmission/initiation) and a teacher provides “pointing-out” instructions to the nature of the mind. I think shikantaza is more similar to vipasanna and is a sutric practice.
John Daido Loori, a former abbot of the Zen Mountain Monastery in New York, talks about some of the basics of zazen. John Daido Loori is one of many influential western Zen teachers that studied under the Harada-Yasutani lineage started by Yasutani Hakuun Roshi (Others were Roshi Kapleau of Rochester Zen Center and Robert Aitken of Honolulu Diamond Sangha).