When I make a table of contents with the article and report classes the results are different. I have two questions.

1. Why does this happen?

2. How can I make the report ToC look like the article one?

Here are a pair of examples:

\documentclass[]{report}
\usepackage{lipsum}
\begin{document}
\tableofcontents
\section{Section}
\lipsum
\end{document}


\documentclass[]{article}
\usepackage{lipsum}
\begin{document}
\tableofcontents
\section{Section}
\lipsum
\end{document}

-

By default, the "top-level" entries in a table of contents will be typeset in bold and without a dotted line between entry name and page number. For the article class, the "top level" is \section, while for the report (and the book) class it's \chapter. Simply changing \section to \chapter in your first example will result in (roughly) the same TOC layout.
2. Consider to use a package to design the table of contents, for instance tocloft or titletoc.
By default, in the report class, the tocdepth counter is set to 2 (so that the "lowest-ranking" heading going into the TOC is \subsection), while in the article class tocdepth is 3 (so that a \subsubsection will make it into the TOC). It seems Leslie Lamport didn't have different TOC layouts for report vs. article in mind. – lockstep Nov 6 '10 at 21:40