Serif, Sans-serif, and Monospaced are categories of 'types of fonts'.

Serif-
-The letters and characters end in strokes.
-Serif fonts are usually used in paragraphs or bodies which
make use of small text as it is easily readable.
-You'll see Serif fonts in the body of a newspaper article

Sans-serif-
-The letters and characters are plain and don't have strokes
at the end.
-Sans-serif is usually used in places where you want to make
the text stand-out
-You'll see this type of font in the title of a newspaper
article
Monospaced-
-In Monospaced fonts each letter and character have the same
width. i.e they occupy the same amount of horizontal space.
-These kinds of fonts are usually seen in command lines.

There may be exceptions