What is difference between PCIe 2.1 x16 and PCIe x16

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My motherbord has a PCIe x16 slot. What is difference between PCIe 2.1 x16 and PCIe x16. Can my motherbord afford PCIe 2.1 x16 or PCIe 3.0 x16 version Graphic cards

[Nacimota]

The newer versions of PCIe are faster, among other things. Newer versions are backwards compatible with the old versions. That is, you can put a PCIe 2.0/2.1/3.0 card in a PCIe 1.0 slot and it will work, but it will be limited to PCIe 1.0 speeds.