A scandoubler converts the Amiga's PAL/NTSC signal (~15 Khz), into a VGA compatible format (~31 Khz).
In essence, it doubles the original horizontal scanrate, since PC monitors can't display anything below 31Khz.
A flickerfixer is an extended version of a scandoubler, it both doubles the scanrate (so a PC monitor can display the Amiga native PAL/NTSC modes) and adds a memory buffer.
This memory buffer is used to 'deinterlace' the interlaced screenmodes, so the don't 'flicker' on the screen anymore.
Ofcourse, the scandboubler won't work on a ECS/AGA non-PAL/NTSC mode (such as DoublePal or Super72).
FlickerFixers might deinterlace those 'extended' modes, but that varies according to model and make.
Hope this helps!
