Here's an amplification on Andrew's comments:

Andrew Huang wrote:

6. -sempai, Sempai
  A coworker or student with more seniority is addressed as "Sempai" or
name + "-sempai" by those with less seniority. It is not used, though,
from employee to boss. "-san" or the appropriate title ("-shachou" for
company president, for example) is used instead.
  As for more senior workers to junior colleagues or upper to
lowerclassmen, "-kun" is used, usually, although "-san" might also be
used. Now, "kouhai" basically means "junior", sort of an opposite to
"sempai" in terms of definition, but "-kouhai" is never used as an
honorific or term of address.

    Senpai/Kouhai is a "system", not just a form of address.  It's also
found in the business world as well as in schools.  When a new
student/coworker comes to the institution in question, they are introduced
to someone who's been around and knows the ropes.  This person becomes their
"senpai", and is responsible for helping them acclimate to the school or
company, and is also the person the rookie turns to in times of trouble.  In
return, the rookie (kouhai) runs errands and helps the senpai where it is
possible.

    Examples of this can be found in "Oh, My Goddess".  When Keiichi is
first introduced, he seems to be Tamiya's kouhai.  Hence, he's home alone,
answering the phone for the frat house at Tamiya's request.  Later in the
manga, Keiichi becomes senpai to a new student, Sora Hasegawa, who runs
errands for him.  (Hasegawa has a cameo in the last part of the OAV,
delivering a note to Belldandy on Keiichi's behalf when the bugs are
preventing them from getting close to each other.)

    BTW, "senpai" is the correct spelling.  When "n" falls before a "p" or
"b", it's pronounced like an "m".

    Ja mata,
    Larry F