It is more like a stamp but always made of jade or stone. And at botom of it, there is the name engraved of the owner or some simple words. When they need to sign their name, they use it instead. And it is a symbol of high class in ancient China. And in ancient China, the stamp of emperor called"玉玺（yu4xi3）", and other people's stamp called 印yin4/印章.
you may refer to some pictures by the website below.