Difference between \if and \ifx

Difference between \if and \ifx

I try to understand the subtleties of TeX programming, but it is not
always easy. For example, in the following code :
\def\first{abc}
\def\second{abc}
\ifx\first\second OK!\else false \fi
I understand why the output is OK!.
But I do not understand why the output is false with this code :
\if\first\second OK!\else false \fi