• DirectX 10.1 is a series of extensions to DirectX 10
• It's supported by upcoming graphics hardware, but not current DX10 hardware
• It requires (and will be part of) Vista Service Pack 1

What are the changes? DX 10.1's goals are to offer the "complete" DX 10, giving developers better control over image quality and making mandatory some of the things that are optional in DX 10. For example, 32-bit floating point filtering is optional in DX10 (16-bit FP filtering is mandatory), but will be mandatory in DX 10.1. Also, in DX 10, the number of multisample anti-aliasing samples is optional—DX 10.1 will make 4x AAmandatory, and require two specific sample patterns. Graphics cards can offer more sample patterns, and developers can query them in their shaders. Graphics cards that are DX 10.1 compliant will have to offer programmable shader output sample masks and multisample AA depth readback. Game developers will be able to index into cube maps and perform bitwise copies from uncompressed textures to block-compressed texture formats.

If that's a bunch of gobbledygook to you, don't sweat it. The main takeaway is this: DirectX 10.1 is a straightforward incremental update to DX 10 that forces graphics vendors to adhere to a few more set standards with regards to image quality and a couple other under-the-hood graphics features, mainly to give games more control over image quality.