This is the preferred sampler for images with lots of small details like blurry effects (DOF, glossy reflections, etc). It also takes up less RAM than the Adaptive subdivision sampler.
Min subdivs - determines the initial (minimum) number of samples taken for each pixel. You will rarely need to set this to more than 1, except if you have very thin lines that are not captured correctly, or fast moving objects if you use motion blur. The actual number of pixels is the square of this number (e.g. 4 subdivs produce 16samples per pixel).
Max subdivs - determines the maximum number of samples for a pixel. The actual maximum number of sampler is the square of this number (e.g. 4 subdivs produces a maximum of 16 samples). Note that V-Ray may take less than the maximum number of samples, if the difference in intensity of the neighbouring pixels is small enough.
Use DMC sampler threshold - when this is on (the default), V-Ray will use the threshold specified in the DMC Sampler to determine if more samples are needed for a pixel. When this is off, the Color threshold parameter will be used instead.
Color threshold - the threshold that will be used to determine if a pixel needs more samples. This is ignored if the Use DMC sampler threshold option is on.
Show samples - if this is on, V-Ray will show an image where the pixel brightness is directly proportional to the number of samples taken at this pixel. This is useful for fine-tuning the antialiasing of the image.
Adaptive Subdivision Sampler
This is an advanced image sampler capable of undersampling (taking less than one sample per pixel). In the absence of blurry effects (direct GI, DOF, glossy reflection/reftaction etc) this is the best preferred image sampler in V-Ray. On average it takes fewer samples (and thus less time) to achieve the same image quality as the other image samplers. However, with detailed textures and/or blurry effects, it can be slower and produce worse results than the other two methods.
The following diagram shows visually the way V-Ray works when using the Adaptive Subdivision image sampler. With this mode V-Ray creates a secondary grid on top of the pixel grid and uses this grid to position the samples. This allows it to use less than a sample per pixel. After the first pass the samples are compared and if the difference between two samples is bigger than the values in the thresholds the grid is subdivided and more samples are added. During the whole time V-Ray has to keep the whole grid in the memory which makes this method less memory efficient compared to the other two methods - see the Notes below.