The --side-by-side (-y) option selects side by side format. Because side by side output lines contain two input lines, the output is wider than usual: normally 130 print columns, which can fit onto a traditional printer line. You can set the width of the output with the --width=columns (-W columns) option. The output is split into two halves of equal width, separated by a small gutter to mark differences; the right half is aligned to a tab stop so that tabs line up. Input lines that are too long to fit in half of an output line are truncated for output.
The --left-column option prints only the left column of two common lines. The --suppress-common-lines option suppresses common lines entirely.