There are two ways (and possibly more) to place content side-by-side in a beamer presentation, the columns and the minipage environments. The first is a beamer-specific environment and is therefore only available in a beamer presentation. Whereas the latter has other applications and is available in all document-classes.
General considerations
In any LaTeX document, there is a predefined width available for text, \textwidth. In order to place content side-by-side, this width has to be split in such a way that the total width of all elements does not exceed \textwidth. Otherwise, LaTeX will automatically break the line and place elements on top of each other, which is usually not desired. The value \textwidth can be used to define the width of a column or minipage. For example, 0.33\textwidth takes a third of width of a page available for content. LaTeX also adds a small horizontal space between elements. Therefore, in order to split a page into three equal parts, 0.3\textwidth might be a more reasonable number. If required, one can add more horizontal white-space between two elements using \quad or \qquad.
The columns environment
The columns environment is only available in the beamer document-class and might therefore be lesser known.
Basic command structure:
\begin{columns}
\begin{column}{0.48\textwidth}
%Content
\end{column}
\begin{column}{0.48\textwidth}
%Content
\end{column}
\end{columns}
And here is a complete minimal working example:
\documentclass{beamer}
\begin{document}
\begin{frame}{The columns environment}
\begin{columns}
\begin{column}{0.47\textwidth}
\begin{itemize}
\item First item
\item Second item
\item Third item
\end{itemize}
\end{column}
\begin{column}{0.5\textwidth}
\rule{\textwidth}{0.75\textwidth}
\end{column}
\end{columns}
\end{frame}
\end{document}
The minipage environment
I wrote an article on the minipage environment quite some time ago. In beamer, the usage is not different.
The command is used as follows:
\begin{minipage}{0.45\textwidth}
%Content
\end{minipage}
Below is a complete minimal working example:
\documentclass{beamer}
\begin{document}
\begin{frame}{The minipage environment}
\begin{minipage}{0.47\textwidth}
\begin{itemize}
\item First item
\item Second item
\item Third item
\end{itemize}
\end{minipage}
\begin{minipage}{0.5\textwidth}
\rule{\textwidth}{0.75\textwidth}
\end{minipage}
\end{frame}
\end{document}
Final note
Even though there are differences, I don’t know of any advantage of one method over the other when placing content side-by-side. However, I suggest choosing one environment and sticking to it throughout a presentation.