WDM stands for Windows Driver Model. It is a method of getting audio from an application through Windows to the audio interface, and vice-versa. WDM goes through what's called the K-Mixer (kernel mixer, an internal several channel mixer inside Windows). At introduction with Windows 2000, ME, and XP, it was to offer much lower latencies and more flexibility than the previous MME audio drivers. This has been proven somewhat true in the pro-audio realm. In the consumer market, WDM seems to work very well.