The snowball effect is present in any process that starts from an initial small state and builds upon itself, becoming larger perhaps potentially dangerous or disastrous (a "spiral of decline" or "vicious circle"), but might be beneficial (a virtuous circle, or often a self-fulfilling prophecy).
The common analogy is with the rolling of a small ball of snow down a snow-covered hillside; the ball will pick up more snow as it rolls, gaining more mass and surface area, and picking up even more snow as it rolls along. In cartoons, this gag is almost a cliche.
See also avalanche, exponential growth, domino effect, butterfly effect
