  Over the last 12 years, AGN monitoring by RXTE, has revolutionised our
understanding of the X-ray variability of AGN, of the relationship between AGN
and Galactic black hole X-ray binaries (BHBs) and hence of the accretion
process itself, which fuels the emission in AGN and BHBs and is the major
source of power in the universe. In this paper I review our current
understanding of these topics. I begin by considering whether AGN and BHBs show
the same X-ray spectral-timing `states' (e.g. low-flux, hard-spectrum or `hard'
and high-flux, soft-spectrum or `soft'). Observational selection effects mean
that most of the AGN which we have monitored will probably be `soft state'
objects, but AGN are found in the other BHB states, although possibly with
different critical transition accretion rates. I examine timescale scaling
relationships between AGN and BHBs. I show that characteristic power spectral
`bend' timescales, T_B, scale approximately with black hole mass, M_BH, but
inversely with accretion rate, mdot_E, (in units of the Eddington accretion
rate) probably signifying that T_B, arises at the inner edge of the accretion
disc. The relationship T_B proportional to M_BH/mdot_E is a good fit, implying
that no other potential variable, e.g. black hole spin, varies significantly.
Lags between hard and soft X-ray bands as a function of Fourier timescale
follow similar patterns in AGN and BHBs. [Abridged]
