Here's how it works.  HER2 is a receptor protein which is formed on the
cell walls to make the cell respond to a growth-factor hormone.  One of
the ways cancer cells can circumvent the normal controls on growth is to
make extra copies of this receptor (or the one for another hormone such
as estrogen).  This is referred to as overexpression.  The pathologists
refer to a tissue as being HER2+ when the amount of overexpression is
enough that Herceptin might work.

Herceptin is an artificial antibody which targets the HER2 protein,
prompting the immune system to attack cells which have a lot of it.

Herceptin can in some people have serious long term side effects such as
heart damage, so it is only recommended for metastatic disease, although
it is used for appropriate severe early-stage cancers in clinical trials
and off-label prescription.  While it doesn't work for patients who are
not HER+, is does not always work for those who are either.  For a
minority it can be dramatically effective.

Other treatment options for metastatic disease are mostly aimed at
symptom control.  Further chemotherapy is an option, but generally not
very effective against bone mets, so most oncologist will want to hold
that in reserve for a time when it can have more benefit.  The main
treatment for bone mets is local radiotherapy, which is very effective
at stopping (but not totally destroying) individual tumours, it
typically sets back their growth by about two years.  This is usually
prescribed as a pain control treatment, again, you get the most benefit
by saving the attack for tumours which are causing problems.