Most positions have "paths" which allow career progression - and help distinguish increasing responsibilities and abilities. Typically this is a combination of industry standard as well as company specific (it's in the interest of everyone to have roughly similar responsibilities associated with titles).
So when comparing
Senior Director there could be different expectations or responsibilities associated with the position even though the "job" is the same. Many of these responsibilities may be difficult to see from an external perspective, as well, but as Chad notes the difference is generally increased responsibilities.
Senior Director position also allows for promotion once in a
Director role (some positions may have pay range limitations, etc) to avoid perceptions of stagnation, etc, especially if someone is a director for a considerable amount of time.
As an example, a more technical career progression in engineering, summarized from this is similar to:
- Associate Engineer
- Senior Engineer
- Staff Engineer
- Senior Staff Engineer
- Principal Engineer
- Distinguished Fellow
Note this is not necessarily exactly the same in all companies.
Academia has its own heirarchy (especially when combined with "Professor" - see here for just some of this).
Also note that "Associate" is used within some engineering as well as academia to effectively represent "junior" - it sounds considerably nicer, however.