What are the things that will be taxed on while passing through customs? Will the tax be upon entering or exiting the country, or both?

Also, I'll be leaving Guangzhou, China for a month and then return; I'll be bringing a laptop with me (not Mac) plus an iPod. I won't be able to show my receipt since I didn't buy the items in China in the first place and both are for personal use. Will this be taxed?