  We briefly review, at a level appropriate for graduate students and
non-specialists, the field of quasar absorption lines (QALs). Emphasis is on
the intervening absorbers. We present the anatomy of a quasar spectrum due to
various classes of intervening absorption systems, and a brief historical
review of each absorber class (Lyman-alpha forest and Lyman limit systems, and
metal-line and damped Lyman-alpha absorbers). We also provide several heuristic
examples on how the physical properties of both the intergalactic medium and
the gaseous environments associated with earlier epoch galaxies can be inferred
from QALs. The evolution of these environments from z=4 are discussed.
