Action is about conflict. Whether with guns, blades, fists, or mysterious powers, these anime feature characters in combat - either to protect themselves or the things or people they value, or simply as a way of life.
Seinen (青年), translated as "youth", is a demographic aimed at men from late teens to adulthood who understand kanji. Seinen manga rarely include furigana with kanji, and have a wide variety of topics and themes. Seinen is the counterpart of Josei, a demographic aimed at women.
Shounen (少年), translated as "young boy", is a demographic aimed at boys and young men, from elementary school age to around the age of 15. Shounen is the counterpart of Shoujo, a demographic aimed at teenage girls.