Sencha is Japanese green tea made from steamed, rolled, and dried tea leaves. The steaming step is important: it stops oxidation soon after harvest and gives sencha its fresh green, grassy, vegetal character. Many Chinese green teas are pan-fired instead, so they often taste nuttier or toastier by comparison.
Sencha is a category, not one single plant variety. It can be made from cultivars such as Yabukita, Saemidori, Okumidori, and others. That is why one sencha can taste crisp and bright while another feels deep green, sweet, and almost brothy.