Using gradients is the easiest way to depart from the lifeless flat look of solid color fills, and as such, gradients are one of the most important features of vector graphics. Designers can create amazingly complex and photorealistic art using nothing but carefully laid out gradients. For all their versatility, gradients are relatively simple to create and edit, render quickly, and are almost universally supported in SVG software—none of which is quite true, for example, for SVG filters such as blur (17.1 Blur).
Basically, a gradient is a smooth transition between two or more colors. Color also includes a level of opacity; this means that you can, for example, make a gradient from opaque red to transparent red, with semitransparent red shades in between. SVG supports two types of gradients: linear (along a line) and elliptic, or radial (away from a center, with possibly unequal axes and a noncentral focus point).