A sans serif font is a basic type of font that has extensions or lines on the ends of the characters?

Serifs are the little flourishes that you see at the ends of strokes on some letters. I'm not talking about the super-fancy flourishes you see in very old documents. Look at different kinds of font, and you will see things like a little anticlockwise curl at the bottom of the letter y, or the little downward strokes at the ends of the letter s. You often see the bottom strokes of upper case letters finished with a short horizontal base. Those are serifs as well. A sans serif font is like the one used in this answer; almost all of the letters are clean and basically free of all the decorative flourishes of some other fonts. Sans is French for without. This font does have some serifs, though, like the curl at the bottom of lower case t.