Kaijin is a humanoid monster whose size, unlike the "Kaiju" (who tend to be very large), is the same as humans. They share characteristics common to humans like walking on their feet, understanding human language, and having evil feelings etc.
Kaijins became incredibly popular in Japan when they started to appear on TV in real-life hero story "Kamen Rider" in 1971. The Shocker and Gel Shocker Kaijins such as "Kumo-otoko (Spider man)" are many of the most famous in the Kaijin history. After the series, there were many sequels made, and also spin-offs and numerous other Kaijin TV series.
Kaijiins can be humans reconstructed (mixed with animals and insects) to have special power, they can be wearing clothes with special power, they can be space aliens, robots, monsters, or beings with all kinds of magic power. They come in all kinds of shapes, colors, characteristics and they are often killed at the end of the episode. But some Kaijins appear many times as regulars on the series if they become very popular.