Module Variations allow for the individual styling of each module within a position, without it being global. Code wise, this is achieved by adding a class into the module code which loads from the Module Class Suffix field in the Module Manager. The theme CSS adjusts accordingly.
Enter any available suffixes at Extensions → Module Manager → Module → Module Class Suffix
There are 13 suffixes: square1-6, title1-3, flush, flushtop, flushbottom & basic.
The Square suffixes represent different shades / coloring of the module variation already present. The Title suffixes change the styling of the module titles only (remove bottom border, change to uppercase and increase font size), and leave the rest of the module styling unaffected. Remember, you can always combine suffixes together such as square6 title1 title3, which will produce a colored module, with a title that has uppercase letters and no bottom border.
Flush will disable all padding and margin to the module for a tighter fit, with FlushTop and FlushBottom disabling the margin/padding for the Top/Bottom parts of the module respectively. Basic removes all image based styling from the module.