Japan is a humid country and mushrooms grow well. Shiitake, shimeji, matsutake, enoki, nameko are the most popular and commercialized mushrooms in Japan. Shiitake, shimeji, and enoki can be found in the United States at Asian grocery stores.
Shimeji can grow big like shiitake, but commercially available ones are always small and sold in a plastic container. Often called buna shimeji or beech mushrooms in the United States. They come in brown or white.
When you use, cut of the connected stumps at the bottom, and cook. They are flavorful but milder than shiitake mushrooms, and versatile for many different cooking. You could eat raw, but have slight bitterness. If you want to add in a salad, saute quickly and mix with other fresh vegetables.