The \hline command will draw a horizontal line the width of the table. It's most commonly used to draw a line at the top, bottom, and between the rows of the table.
Two successive \hline commands leave a space between the lines; this space does not contain the vertical lines specified by | characters in the col argument of the \begin{tabular}{col} command.
To place a line at the bottom of a table, the \hline command must follow a \\ (line break) command after the last table entry. This final line break command should be present only in this circumstance.