  This overview provides a historical perspective highlighting the pioneering
role which the fairly modest observational facilities of ARIES have played
since the 1990s in systematically characterizing the optical variability on
hour-like time scale (intra-night optical variability, or INOV) of several
major types of high-luminosity Active Galactic Nuclei (AGN). Such information
was previously available only for blazars. Similar studies have since been
initiated in at least a dozen countries, giving a boost to AGN variability
research.
  Our work has, in particular, provided strong indication that mild INOV occurs
in radio-quiet QSOs (amplitude up to $\sim 3-5\%$ and duty cycle $\sim 10\%$)
and, moreover, has demonstrated that similarly mild INOV is exhibited even by
the vast majority of radio-loud quasars which possess powerful relativistic
jets (even including many that are beamed towards us). The solitary outliers
are blazars, the tiny strongly polarized subset of powerful AGN, which
frequently exhibit a pronounced INOV. Among the blazars, BL Lac objects often
show a bluer-when-brighter chromatic behavior, while the flat spectrum radio
quasars seem not to. Quantifying any differences of INOV among the major
subclasses of non-blazar type AGNs will require dedicated monitoring programs
using $2-3$ metre class telescopes.
