To what extent can we see signals that fall between frequency bins?
zero-padding will not "improve resolution" in the sense that it would help you separate two peaks in the spectrum that are very close (too close) to each other.

but zero-padding, as well as interpolation in the frequency domain, does improve resolution in the sense that it help you to more precisely estimate the location of an isolated peak in the spectrum.  in fact, zero-padding in the time domain is equivalent to interpolating in the frequency domain using the sinc() function (actually the dirichlet function) as the interpolation kernel.