These are notes based on a series of lectures given at the KITP workshop
"Quantum Criticality and the AdS/CFT Correspondence" in July, 2009. The goal of
the lectures was to introduce condensed matter physicists to the AdS/CFT
correspondence. Discussion of string theory and of supersymmetry is avoided to
the extent possible.Comment: 64 pages, many figures; v2: references, clarifications; v3: minor
correction
