Voltage pushes charge round a loop, resistance holds it back, and the current that survives the argument is what makes light and heat. Build a circuit below and the page solves it properly — the same node equations an engineer would write — then shows you the charge moving, faster where the current is bigger.
For a fixed resistor, doubling the voltage doubles the current: the V–I graph is a straight line and its gradient is the resistance R, measured in ohms (Ω). Plenty of things do not behave like that. A filament lamp gets hotter as you push more current through it, its tungsten wire resists more when hot, and its graph bends. Both are below — move the sliders and watch the gradient.