Lines are compared with the Myers diff algorithm (the one git diff uses). Inside a changed line, the words that differ are highlighted.
Ignore whitespace treats a = 1 and a=1 as the same line. Ignore case treats Foo and foo as the same.
Long unchanged stretches are folded to 3 lines of context — click a fold to show it. Copy diff gives a standard unified diff (---/+++/@@ hunks) you can paste into a review or apply with patch.
You can drop a file onto either box. Shortcut: Ctrl/⌘+Enter copies the unified diff.