Each version of Direct X offers newer features and better rendering - in other words, prettier pictures that are more detailed and realistic (assuming your PC has the hardware to handle the heavier load).Up until version 9.0c, the latest version of Direct X was generally available as a separate download from Microsoft.Direct X 10 was only available for Vista and not XP.
Direct X is the most important interface between the graphics card and a computer game.
However, from version 10 onwards, the latest update for Direct X is available only for specific operating systems and under very controlled circumstances.
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However, a game written specifically for Direct X 10 will generally not run on Direct X 9 or earlier.
It allows developers to standardise certain video display and audio files - this makes them easier to develop and run on different types of PCs (it is also used on consoles such as the Xbox). So, if a game was written for Direct X 9, it should run if you have Direct X 10 or Direct X 11 installed on your PC.