A sans serif font is, as the name implies, a font without serifs. In typographic terms, serifs are the small strokes or extensions at the end of a longer stroke, such as the leg of a “K” or “R.” There are several styles of sans serif fonts, including geometric sans serifs like Futura, humanist sans serifs like Frutiger, grotesque sans serifs like Franklin Gothic, and neo-grotesque sans serifs like Helvetica. Sans serif fonts are popular with brands, and are usually easier to read on screens because they have a simpler design than serif fonts.
Sans serifs are generally easier to read on small screens like smartphones and smartwatches. Since they have fewer details and features, they are easier to render clearly in those environments. Serif fonts are generally easier to read in more traditional reading environments such as books and even longer-form web formats. Serif fonts offer more visual distinction between the different letterforms, which makes it easier for the eye to interpret the text.
Absolutely, it’s very common to pair sans serif and serif font, especially in publishing situations where a sans serif might be used for headlines and a serif used for text. This is also common in branding. The key is to find a sans serif and serif that complement each other, which requires experimenting with a lot of different combinations. That said, good pairing is relative to your design goals, and you can pair mismatched sans serifs and serifs to create a jarring, disjointed look if you want.
You can really use sans serif fonts whenever you want. They are one of the most versatile styles of fonts available to designers, and the style encompasses many different sub-genres. They can be used in logos, long-form online text, UX copy, books, magazines, advertisements, and so on. The trick is to use the right sans serif for the purpose. Humanist sans serif fonts, for example, generally deliver the better legibility than other sans serif styles. Geometric sans serifs are very popular for use as logos and in branding, but their design features a lot of similarly shaped letterforms, which hampers legibility. So, use san serifs anywhere, but choose them thoughtfully.
You can pair two sans serif fonts, technically, but not as easily or successfully as pairing a sans serif with something different. Pairing fonts is part art and part science, but the goal is usually to find two or more fonts that complement each other while also bringing visual interest to a design. Pairing two similar fonts defeats both purposes. So is it possible to pair two sans serif fonts? Sure, especially if you find two that are distinct from each other, such as a bold or eclectic display sans serif and a more refined sans serif for text.