One holds voltage steady no matter what you connect to it. The other holds current steady instead. Neither ideal actually exists — and the gap between ideal and real is exactly why your headlights dim when you crank the engine.
An ideal voltage source maintains a fixed output voltage regardless of how much current the load draws — until it physically can't, because it's hit its maximum current capability. An ideal current source does the opposite: it maintains a fixed output current regardless of the load's voltage or impedance — until it hits its compliance voltage, the maximum voltage it can produce to force that current through. Real sources are neither. Every real voltage source has internal impedance that makes its terminal voltage sag under heavy load; every real current source has some finite output impedance too, which is exactly why the "ideal" version of each is a modeling convenience, not something you can buy off a shelf.