inertial forces

it was mentioned in another thread that only inertial forces are frame dependent or relative.

what is an inertial force?
what are the differences between inertial and non-inertial forces?

and most importantly:
why inertial forces are frame dependent while other forces are not?

i tried wikipedia but they never give a clear picture there.
iam still a beginner at relativity so i hope to get full explanation here.