These are active elements and are of two types of voltage and current sources,
Independent sources -

• The value of source does not depend on the condition of the circuit .E.g. Voltage Source and Current Source

• Dependent sources - The value of source depends on the condition of the circuit. E.g. Voltage controlled voltage source (VCVS), Current controlled voltage source (CCVS), Current controlled current source (CCCS), Voltage controlled current source (VCCS). 

Concept of Voltage and Current Sources:

Ideal Voltage Source:

• The voltage source which generates voltage with respect to given time variation i.e. which generates voltage with respect to time change.

• Neither voltage magnitude nor time variation changes with magnitude of output current.

• If in some manner the terminal voltage is made equal to zero, the source behaves as a short circuit.

• The ideal voltage source does not have resistive, inductive or capacitive effects.

Ideal Current Source:

• The current source which generates current with respect to given time variation i.e. which generates current with respect to time change.

• Neither current magnitude nor time variation changes with load.

• If the output current of the ideal source is adjusted to be zero, the source behaves as an open circuit.

• Unlike voltage source, the ideal current source also does not have resistive, inductive or capacitive effects.