An ideal voltage source has a fixed voltage, and can source -or- sink any current without the voltage changing.
An ideal current source can have a voltage across it of any value and any polarity, but puts out a fixed current.
So in parallel without a load, the current source will simply flow through the voltage source. Put any load on it, and the current through the load will be V/R = I. Some or all of the current may come from the current source. All that the external load sees is a constant voltage and a current based on that voltage and the load resistance.
The situation is reversed if you place them in series - an ideal voltage source and an ideal current source in series act as just an ideal current source.
