You can make tables using \begin{array}{colspec} … \end{array}, where colspec is some string consisting of l, c, r specifying left, center, and right alignment for the columns. The colspec may also have | characters indicating the placement of vertical column-separating bars.
To draw horizontal row-separating lines, use the \hline command.
Here is a moderately complex example:
$$ \newcommand{smallm}[0]{\overset{n\ \gg\ m}{\longrightarrow}} \begin{array}{|l|c|c|} \hline \\ & \textrm{Straightforward approach, done well} & \textrm{Fenwick tree} \\ \hline \\ \textrm{Space} & O(m + n) \smallm O(n) & O(m) \smallm O(1) \\ \hline \\ \textrm{Time} & O(m + n) \smallm O(n) & O((m + n) \log m) \smallm O(n) \\ \hline \end{array}$$
Here is a very complex example of a truth table:
$$\begin{array}{ccc|c|c} \text{*p == *q} & \text{*q != '\0'} & \text{*p != '\0'} & {\begin{array}{l}\text{*p!='\0' &&}\\\text{*q!='\0' &&}\\\text{*p==*q}\end{array}} & \begin{array}{l}\text{*q != '\0' &&}\\\text{*p == *q}\end{array} \\ \hline \\ \begin{array}{c}T\\T\end{array} & \begin{array}{c}T\\T\end{array} & \begin{array}{c}T\\F\end{array} & \begin{array}{c}T\\F\ \textrm{(impossible)}\end{array} & T \\ \hline \\ \begin{array}{c}T\\T\end{array} & \begin{array}{c}F\\F\end{array} & \begin{array}{c}T\\F\end{array} & \begin{array}{c}F\ \textrm{(impossible)}\\F\end{array} & F \\ \hline \\ \begin{array}{c}F\\F\end{array} & \begin{array}{c}T\\T\end{array} & \begin{array}{c}T\\F\end{array} & \begin{array}{c}F\\F\end{array} & F \\ \hline \\ \begin{array}{c}F\\F\end{array} & \begin{array}{c}F\\F\end{array} & \begin{array}{c}T\\F\end{array} & \begin{array}{c}F\\F\end{array} & F \\ \end{array}$$