Can somebody explain PCI Express slots for me?

I’m building a computer and I need to know what will fit in what when I’m purchasing parts. What’s the difference between PCI express 2.0, PCI express 2.1, PCI express x16, etc. Will a express PCI 2.1 plug on a video card fit into a PCI express 2.0 slot on a motherboard?? I GREATLY appreciate [...] http://bit.ly/pmitdu