A slab serif font is a serif font where the serif is squared off, giving the font a blocky, sturdy appearance as opposed to the more refined look of a traditional serif. Slab serifs tend to have less contrast overall than other serifs, which contributes to their heavy, often dramatic appearance. Slab serif fonts are typically used where a display font is required, such as book covers, posters, advertisements, logotypes, or as complimentary fonts in a broader type system. Popular slab serif fonts include Rockwell, Clarendon, Serifa, Courier, and Memphis.
Slab serif fonts are typically used for display purposes--headlines, posters, billboards, logos, and other situations where bold type can stand out. Their sturdy structure and often exaggerated design accents are excellent at grabbing attention. These same features make many slab serifs a less than ideal choice for long-form copy like magazine articles, though there are slab serif fonts that work well in blurbs and text of moderate length.
The difference between a serif font and a slab serif font lies in the serif itself. On a traditional serif font, the serif is usually angular, with either a curved or diagonal edge leading to a pointed end of the serif. On a slab serif font, the serif is almost always squared off, giving the letterform a blockier appearance. The result is that regular serif fonts tend to have a more delicate or refined look, while slab serif fonts are heavier and more commanding.