Line break in the table item

LaTeX needs to know where it should break the line, so you have to specify the width of the cell. You could use p columns, such as:

\begin{tabular}{p{3cm}...}

Similar to a parbox of 3 cm width, the text is automatically wrapped and left aligned. Even more, it's fully justified.

Here's an example:

\begin{tabular}{|l|c|r|p{1.7cm}|}
  \hline
left & centered & right & a fully justified paragraph cell\\
\hline
  l & c & r & p\\
  \hline
\end{tabular}

Note: those separation lines are just for showing the cell dimensions, I don't recommend to use such lines in tables, especially not the vertical ones.

You can see this and further examples of tables and explanation here: Creating tables with LaTeX.