枕头 versus 垫子, are these different? I've come across these two words in a picture-book. Are  枕头 and 垫子 the same kind of pillow or are they different kinds of pillow?
枕头(Zhen Tou) is the pillow which is on your bed. That's the only meaning of that word.
However, 垫子(Dian Zi) usually means a mat or pad which you are sitting on. Or means some pad could be put under a glass cup, for instance.