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".
This is easily the most relaxing/laid back series I have ever seen. I typically watch action shows, but Mushi-shi was engaging enough for me to enjoy the entire series. Since the series is more like a collection of short stories rather than one coherent story, the episodes don't end with cliffhangers and I didn't stay up another hour to finish a mini-arc like I do for some other series. Thus, if I had a complaint with the series it would be that there were times in which I wanted to see old characters again or get at least a small amount of closure from the story. The flip side to that coin is that the show is great if you want to watch a few episodes here and there without getting sucked into the plot and losing your Saturday afternoon. I might describe the feel of the show as an anime version of Planet Earth or something to that effect. If you appreciate aesthetics and want a chill series, definitely watch Mushi-shi... if you have ADHD and need an explosion every two seconds then you should try a different show.