Active vs Passive DisplayPort DVI adapters
An active adapter converts a true displayport signal into DVI/HDMI/VGA/whatever else. It will have either a converter chip or a DAC built in to do said conversion. Contrary to popular belief, they do not need to have a usb power connection, in fact there are many that do not.
Passive adapters are merely a cable to go from displayport to DVI/HDMI/VGA. The video card detects the monitor as such, and outputs the required signal (DVI/HDMI/VGA). DVI/HDMI requires a special clock, which the graphics card is required to provide and may can only provide one or two of. Displayport does not use this clock.
Active adapters, on the other hand recieve a displayport signal from the video card, and convert it to DVI/HDMI/VGA. They will always have some sort of onboard DAC or video converter chip which they use to convert the DP signal and generate the clock