Blocks in Beamer

Question

I want to put boxed text/block in Beamer and my LaTeX source is

\documentclass[pdf]{beamer}
\begin{document}
\begin{frame}
\begin{block}{Observation 1}
Simmons Hall is composed of metal and concrete.
\end{block}
\begin{block}{Observation 2}
Simmons Dormitory is composed of brick.
\end{block}
\begin{block}{Conclusion}
Simmons Hall $\not=$ Simmons Dormitory.
\end{block}
\end{frame}
\end{document}

This results in whereas what I want is

Answer 1

Your MWE is using the default beamer theme, which doesn't define the blocks to have the colors you want. Try using a different theme, as below. Also, to get blue/green/red use block/exampleblock/alertblock, respectively.

\documentclass[pdf]{beamer}
\usetheme{Copenhagen}
\begin{document}
\begin{frame}
\frametitle{Frame title}
\begin{block}{Observation 1}
Simmons Hall is composed of metal and concrete.
\end{block}
\begin{exampleblock}{Observation 2}
Simmons Dormitory is composed of brick.
\end{exampleblock}
\begin{alertblock}{Conclusion}
Simmons Hall $\not=$ Simmons Dormitory.
\end{alertblock}
\end{frame}
\end{document}

You can also customize how things look by using \useinnertheme{} and \usecolortheme{}. See the beamer documentation for more info.

Answer 2

You need to use a theme to have blocks appearing this way. Here is an example with "Boadilla"

\documentclass[10pt,a4paper]{beamer}
\usepackage[utf8]{inputenc}

\usetheme[secheader]{Boadilla}

\begin{document}

\begin{frame}{Title Frame}
    Introduction sentece
    \begin{block}{Title Block 1}
    {
        \begin{itemize}
        \item item 1
        \item item 2
        \end{itemize}
    }
    \end{block}
    \begin{exampleblock}{Title Block 2}
    {
        Text.
    }
    \end{exampleblock}
    \begin{alertblock}{Title Block 3}
    {
        You
    }
    \end{alertblock}    
\end{frame}

\end{document}

giving the result :

You need to use block, exampleblock and alertblock.