\documentclass[landscape]{slides}

\def\content{
    \invisible{Invisible}
    \visible{Visible}
    \begin{itemize}
        \item first
        \item second
        \item third
    \end{itemize}
}

\begin{document}

\begin{slide}
    This is the first slide.
    \content
\end{slide}

\begin{slide}
    This is the second slide.
    \content
\end{slide}


\begin{overlay}
    This is the first overlay.
    \content
\end{overlay}

\begin{overlay}
    This is the second overlay.
    \content
\end{overlay}


\begin{note}
    This is the first note.
    \content
\end{note}

\begin{note}
    This is the second note.
    \content
\end{note}

\end{document}