Examples

diff -U 0 oldFile newFile

or compare corresponding files in both directories, in alphabetical order.

diff -U 0 oneDirectory anotherDirectory

Options

-B = Ignore changes that just insert or delete blank lines.
-b = Ignore changes in amount of white space.
-r = When comparing directories, recursively compare any subdirectories found.
-q = Report only whether the files differ, not the details of the differences.