PCI-X and PCI-Express

Has anyone found a good resource that explains the difference between these two standards or indeed could explain it themselves?

I've been trying to research the topic but after looking through approximately 10 search results in Google I've found conflicting information.

Some talk about PCI-Express being a graphics card only interface with PCI-X being using for other components like Gigabit Ethernet NIC's and Soundcards.

Some say PCI-X is backwardly compatible but then others say PCI-X and PCI-Express are the same thing and are not backwardly compatible

:confused: :confused: :confused:

Someone please so kind as to clear up my confusion :)