Serif : 
1.A serif is a small line attached to the end of a stroke in a letter or symbol.
2.Serifs are small decorative flourishes on the ends of some of the strokes that make up letters and symbols.

Where We Use:

Serif font is used for Printed works.Because,Easier to read.

Ex:Printed Books, News Papers and Magazines.

Serif Fonts Are:
1.Georgia.
2. Palatino.
3.Times New Roman etc..


Sans-Serif:
A Sans-serif means a letter or symbol without strokes.

Where We Use:

Sans-serif  is used in Digital and Web related.

Ex: Websites,Digital marketing.

Sans-Serif fonts:

1.Arial.
2.Helvetica.
3.Tahoma etc..