How to insert manual line breaks inside LaTeX tables?

Usually, you use a column definition like p{3cm} instead of l, and then use \newline instead of \\ in the cell body.

You could do it like this:

\documentclass{report}

\begin{document}

\begin{tabular}{|l|l|}
  \hline
  A & B \\ 
    & C \\ 
  \hline
  D & E \\
  \hline
\end{tabular}

\end{document}

which produces: