Typeface, Font, Serif and Sans Serif

2) The difference between the typeface and a font is that a typeface is a family/group of fonts, whereas a font is more of a specific style and size of the characters.

Typeface is the type design for a set of fonts, and variations of this design form is called the typeface family. Often the set of fonts that are within a typeface all share common design features.

Font includes the style, size and spacing of a set of characters.

3) The difference between serif and sans serif faces is that serif fonts have small tails coming from the starts and ends of the characters. On the contrary, sans serif does not include this design feature and therefore joins all the way around the characters.

For example here is the difference between them visually. I took these screenshots from Google images.

As you can see the serif face has feet and the width of the font varies.

In sans serif it does not have these feet and is the same width throughout.