Serifs are the small lines on the edges of letters. Sans-serif is a typeface that does not have these.
Serif fonts are easier to read in printed works because the serif makes individual letters more distinct and easier to recognize quickly.
Sans-Serif fonts are better for web and mobile.
Printed works usually have a resolution of between 300-1000 dpi (dots per inch) where computer screens are more around 100
Serifs are used to increase contrast and spacing between different letters and improve identification and legibility. They're generally used for body text because they're less likely to cause eye strain.
Sans serifs are better at smaller sizes because the font can stay legible longer through reproduction and smearing because of simple forms.
In my personal opinion, serif fonts are more used for long printed text copy, forms and places in printed works where there would be a lot of text. Sans serif is more used for web, logos, and other graphic design uses.