Need help on understanding hardware detection

Hi All: I did some search on "how does linux kernel detect hardware?" and got what udev is. I konw Linux kernel will send a netlink message to udevd. But I still do not understand how the kernel can decide to send a netlink message. What happend before the netlink message? Take an usb stick for example, what will happen and what the kernel do from the very beginning of it been plugged in? Thank you for advance. Any references will be appreciated. -------------- Tao Jiang (姜涛) 2013-08-19