They are neither plants nor animals. They differ from other forms of life such as the micro-organisms and the fungi. Instead they resemble the primeval body of life and are generally known as "Mushi". Their existence and appearance are unknown to many and only a limited number of humans are aware of them. Ginko is a "Mushi-shi" who travels around to investigate and find out more about the "Mushi". In the process, he also lends a helping hand to people who face problems with supernatural occurances which may be related to the "Mushi".
If you are going into this anime, rearing for a good, long plot and lots of action and fight sequences, don't watch this anime. Each episode is a different story, and there are no main characters other than Ginko. This is a story of his travels to help people with mushi, bug ghost things. This anime has little to no action sequences, and it is mostly talk, but it is simply amazing with the way it draws you in with its animation and the subtle way it makes you fall in love with the main character. This is definitely worth watching, and it was entertaining to the very end, even if it was mostly talking.