This thesis focuses on four diverse topics, whose common denominator is the variability revealed by the large scale surveys: i) development of a classified variable star catalog with an unprecedented depth, area coverage, variety of classified variable star types and purity of classification, at the time; ii) an analysis of an extreme coronal-line emitter caused by a tidal disruption of a star by a supermassive black hole; iii) calibration of a new stellar distance estimator applicable at galactic and extragalactic scales and iv) development of photometric pipelines used in the follow-up networks for the Gaia satellite.