Serifs are theose little extra squiggles on a letter.
Size increase for clarity
Size increase for clarity
The first "Size..." line is a serif font. The second is sans serif (ie.without serif) Notice the extra little lines on the capital ess and the lower case eye? See the little feet on the lower case enn?
Those are serifs. They make text easier to read. Courier has serifs.
So does Times New Roman.