\documentclass{article}

\begin{document}

\section{First numbered section}

Hello, World!

This is \textbf{extremly} interesting.

\begin{section}{Second numbered section}
And again, hello, World!

\begin{bfseries}
This whole paragraph will be typeset in bold.
\end{bfseries}

{\bfseries As well as this one.\par}

\end{section}

\end{document}