Unified mode displays lines from both versions in the same column.  Side-by-Side shows each file’s contents in its own column.

This turns on line wrapping in the file content columns.

This setting controls the number of lines that are the same in both versions which are displayed around changed lines.

These two dropdowns control the version treated as the left and right version in the diff.

Clicking the repository path will take you to the history page for the file.  For each version in the diff, clicking on the File link will download that version of the file.  Clicking on the Details link will load the Transaction Details page for the transaction that created the version.  In the file diff area, changed lines are shown in blue in Side-by-Side mode and red and green in Unified mode.  Clicking on the up/down arrows in the leftmost column will allow you to jump between changed regions.