Undead are beings that have come back to life after dying.  
They are not resurrected however, but return in a semi-form,
their spirits still alive, their bodies still dead (or sometimes,
just their spirits).  What brings them back varies.  Sometimes
it's sheer anger at not being buried, at their manner of death,
and sometimes they're just so mean people that they can come
back to partial life by will alone.  Sometimes, however, it's
out of a need to perform unfinished business, and in this 
regard not all undead are evil.  Undead can also be raised by
deities to perform whatever duties necessary (often guard
duty) and thus it is not unlikely to find the good-aligned
priests and guards of an ancient tomb still carrying out their
duties.

Remember, undead don't drink, sleep, breathe, eat, excrete, or
perform any other bodily function, except for the type that eat
your brains.

Never make the mistake of assuming all undead are evil!  It will
be the last one you make...