Changing a regular parameter only affects new rings. A global parameter corresponds to a regular parameter, and has the same effect, except that it operates on all existing rings at once. For example, Global Aspect Ratio is identical to Aspect Ratio, except that it stretches the entire image, not just new rings. The global parameters are accessed in the Parameters dialog by selecting Edit/Global Params.
A global parameter doesn't override its corresponding regular parameter; rather, it adds to it. Thus global parameters can be combined with regular parameters, e.g. global parameters can modify the behavior of patches. Global parameters aren't saved in patches, but they are saved in playlists.
Global parameters have their own oscillators, which are independent of the regular parameter oscillators. A given parameter can be modulated by both oscillators simultaneously: one affects new rings, while the other affects existing rings.
Regular parameter changes propagate gradually, depending on the rate of Ring Growth, which causes the output to lag behind the parameter changes. This lag (AKA hysteresis) is essential to Whorld's look, but it has a down side: if you try to control regular parameters via MIDI, the controls can feel sluggish because they're not having enough impact. Since global parameters affect the whole image at once, they give a stronger impression of realtime control.
When global parameters are controlled via MIDI, they are affected by damping. This allows you to make a tradeoff between responsiveness and smoothness. It's particularly useful for avoiding jerkiness when controlling global parameters with a noisy MIDI device.
Most of the parameters have global equivalents; the only ones that don't are Ring Growth, Ring Spacing, and the six color parameters. Global hue can already be controlled via mouse and MIDI, and the background color is global by definition.