Weight 

Weight refers to the thickness of a font within a typeface. Thin, regular, semibold, bold, and black are examples of weights. 

Contrast between weights is essential when combining typefaces. Combining a very thick font with a very light one often feels unbalanced. But combining typefaces that are the same weight can also be a challenge. 

Instead, especially when starting out with font combinations, finding typefaces that have noticeable but not extreme differences in weight is easiest. Designers can branch out from there into more prominent distinctions. 

The weight of typefaces has a heavy impact on how well they pair together. Too much difference can make the lighter typeface practically disappear. 

Style 

While style is sometimes used interchangeably with classification, in this case, it’s referring to a font either being normal, italic, or oblique. 

When combining fonts where different styles are used, it’s important to make sure that the italic or oblique styles work as well together as the normal style.

Sometimes italic styles are vastly different from the normal style of the same font, which can make fonts that would otherwise work well together suddenly clash.