From my personal experience of studying the Tea Ceremony, I would say that it is the lived experience of zen. (so now go study zen) But to me it is the religion of beauty--it is the worship of everything temporal--and the discipline of seeing the true beauty in that which passes, like the flickering orange embers of coals or the whisping eddies of steam from the brazier. As far as it's importance to Japanese culture--the tea ceremony is basically the culmination of Japanese Culture because encompasses knowledge of kimono, calligraphy, flower arranging, food prep., gardening, poetry, pottery, history of the fine arts--it's basically all there.