\usepackage{} -- Caption

\usepackage[hang, small,font=sf,labelfont=bf,up,rm,textfont=it,up]{caption} % Custom captions under/above floats in tables or figures  _NEED REF_

...

hang indicates hanging indent (I think, because I've not found this exact usage indicated in any documentation yet).

small indicates the relative font size.  Something somewhere mentioned that using relative font size is preferable to an absolute font size -- I'm not sure why, and I'm certain that some journals and thesis departments require a particular font size, e.g. 10pt.

font=sf defines the fonts for the labelfont and textfont entirely.
    sf is sans-serif. 

labelfont=bf,up,rm defines the label, number, and separator, e.g. Figure 1:
     They will be (in this case) bold, upright, and Roman.  Roman is the default, however since I defined my captions as sf, rm here makes the labelfont serif.

textfont=it,up defines the font of the caption text, in this case italic and upright.