Moore's law is the observation that the number of transistors in an integrated circuit doubles about every two years. Moore's law is an observation and projection of a historical trend.

It is.

It's about the shrinking in integrated circuit feature size, hence increase in IC element densities, so it applies to memories which are integrated circuits such as flash memory and the various kinds of RAM, but not to magnetic storage such as in HDDs as they're something else altogether.

That said, I believe (but am not absolutelly sure) that IC feature size has been shrinking slower than Moore's Law predicts for maybe a decade as the size of the features becomes so small that quantum-level effects start becoming a problem (think stuff like signal leaks due to quantum tunnelling).