TODO: still under construction
The plain driver is the one available as stable release. It is robust, has been widely tested and is included in several Linux distributions.
It was started at a time when there wasn't any generic wireless stack in the kernel, so it implements its own "homegrown" stack as many other drivers did at the time.
However, one of its major drawbacks is that WPA is not supported. It is due to this and especially because of the trend in the kernel to implement a generic wireless stack, that there have been efforts to port the plain driver to this generic stack. See SoftMac and Mac80211 for further information on those.