Current Sources:

Ideal current source is an source which will give an constant current at its terminals irrespective of the voltage across the terminals

Symbol and current source with load is shown in fig:

At any time the value of the current flowing through the load will remains same irrespective of the voltage across the terminals

But practically the current source is having a very high internal resistance as shown in fig with the current source.

Because of Rsh current through its terminals decrease slightly with increase in voltage across the terminals.

For ideal current source Rsh should be infinity.

As we are having the time varying sources for voltage same we are having for the current sources as well:

Time Invarient Sources:

These are also called as DC sources and in which current is not varying with the time so these are called as time invariant

Time Variant sources:

These are also called as AC sources and in which current is varying with the time so these are called as time variant.