The GI cache records the lighting at many points across the image. These are the dots you see in the pre pass; each dot corresponds to one sample. Each sample stores light from many directions.
GI cache detail affects the spacing of the samples in image space during the pre pass. Then, in the final pass the lighting at any part of the scene is a weighted average of nearby samples, or in other words a filtered interpolation. The lower the detail, the coarser the approximation. This usually shows itself as a blurriness or lack of clarity in the GI. However,
GI surface details (GISD) is designed to try to compensate for this lack of detail, but that too is only an approximation even though it has detail down to the subpixel level. Low GI cache detail can also cause some parts of the scene to be completely missed in the pre pass, and this could result in no GI (black) in those areas. This is more likely to be a problem with vegetation or objects that are small and sparse in the image. (This can happen even at high GI cache detail too, but is less likely.)
At each sample point, many rays are cast outwards in a sphere to capture the lighting at that point.
GI sample quality affects how many rays are cast outwards from each sample point. The actual number of rays is much greater than the setting (usually at least 6x), but in general a higher setting for GI sample quality results in more rays. If there's a lot of contrast with respect to the direction the GI is received from, higher sample quality may be needed to avoid what I would describe as blotchiness or completely missing important sources of light. The default of 2 is very low and I would recommend at least 6 for reasonable quality architectural renders. It also affects how much data is stored at each sample. Therefore both GI cache detail and GI sample quality affect the size of the GI in memory and the amount of time taken to render the pre pass. GI sample quality also affects render time for the final pass much more than GI cache detail does, as that primarily affects the pre pass time.
If you're looking for V-Ray-quality architectural GI you'll find that TG 4.0 falls short, but it's not bad for a lot of scenes where you don't need that much detail. If that's what you want from TG, then look forward to Brute Force GI (Path Tracing) coming to Terragen 4 in 2017. That will make a big difference.
Matt
