pdflatex work and latex not work

Question

I am trying to understand the following problem: the "pdflatex main.tex" command runs successfully, but the "latex main.tex" command exits with error. Why does pdflatex work and latex not work? Please see the example below:

\documentclass[a4paper,12pt]{report}
\usepackage{graphicx}
\begin{document}
    Consider the sine function, whose graph is shown in figure (\ref{fig:figure1}).
    \begin{figure}
        \centering
        \includegraphics[width=0.7\linewidth]{Figure_1.png}
        \caption{Sine(x) function.}
        \label{fig:figure1}
    \end{figure}
\end{document}
% [terminal ~]$ latex main.tex
% LaTeX Error messages Line 7: 
% Cannot determine size of graphic in Figure_1.png (no BoundingBox).
% [terminal ~]$ pdflatex main.tex
% Successfully executed!