system
A chat on subreddit /r/pcmasterrace
human
What's the difference?
gpt
An active DisplayPort adapter will do the conversion itself, treating the GPU side like it's DisplayPort and converting it into HDMI on the monitor side. You also need an active adapter for DisplayPort/HDMI to VGA, since the former standards don't natively carry an analog signal. The passive DisplayPort adapter doesn't do any conversions on its own, just the bare minimum to send a digital signal from DisplayPort to HDMI or DVI, and the GPU still sees it like it's HDMI or DVI. 

If you need more than 2 monitors, you need to use either DisplayPort, or active DisplayPort adapters, because DisplayPort ports are able to share the digital clocks the GPU uses to keep connections running. 

On a GPU, it only has the bare minimum clocks available for all the intended inputs on them, and for a GPU with 1 HDMI port and 3 DisplayPort ports, it only needs 2 digital clocks since HDMI ports can't share digital clocks with each other, but up to 6 DisplayPort ports can share 1 digital clock with each other. 

TL;DR:

The difference between active and passive adapters is that there's no signal conversion chip in the latter. 

For 3+ monitors, you need DisplayPort/active DisplayPort adapters because they can all share 1 digital clock on the GPU. Most only have 2. Learned this the hard way.