What are dependent (controlled) sources and how do they affect circuit analysis?

Dependent sources have output controlled by a voltage or current elsewhere in the circuit. Four types exist: VCVS (voltage-controlled voltage source), VCCS (voltage-controlled current source), CCVS (current-controlled voltage source), and CCCS (current-controlled current source). They model transistor and amplifier behavior. Unlike independent sources, they cannot be zeroed during superposition and require modified nodal/mesh analysis techniques.