Here is an app note from Texas Instruments describing the theory of operation of linear regulators. It goes on to explain and compare various topologies used in integrated linear voltage regulators.
The explosive proliferation of battery powered equipment in the past decade has created unique requirements for a voltage regulator that cannot be met by the industry standards like the LM340 or the LM317. These regulators use an NPN Darlington pass transistor (Figure 1), and will be referred to in this document as NPN regulators. The demand for higher performance is being met by the newer low-dropout (LDO) regulators and quasi-LDO regulators.