Zen is the Japanese name for the form of East Asian Buddhism that is known as Chan in China, Seon in Korea, and Thien in Vietnam.
This form of Buddhism orginated in China (as Chan) during the 6th century, and was later transmitted to Korea, Japan and Vietnam.
Since the Chan/Zen form of Buddhism was popularized in the West by Japanese teachers, this form of Buddhism came to be known as Zen in the West. Hence, Westerners are generally more familiar with the Japanese term Zen than the Chinese term Chan, or the eqivalent Korean or Vietnamese terms. For this reason, Western scholars will sometimes use the term Zen as a collective term to refer to Chan/Zen within China, Korea and/or Vietnam.
For example, scholars sometimes refer to "Chinese Zen", "Korean Zen" or "Vietnamese Zen". These are just different names for Chinese Chan, Korean Seon, or Vietnamese Thien, respectively.
All of the Chan/Zen traditions trace their origins back to the Chinese Chan school, and all recoginize a lineage of teachers that includes Bodhidharma, who is credited with establishing the tradition in China, and Huineng (the Sixth Patriarch), whose teachings are recorded in the seminal treatise, The Platform Sutra of the Sixth Patriarch. Sepaerate branches of this lineage were established in Korea and Japan, after Chan/Zen became established in those countries.
Generally speaking, Chan/Zen emphasizes direct insight into the nature of reality that is developed primarily through combining the practice of sitting meditation with a direct mind-to-mind transmission from master to disciple.