Questions about multiple colors or channels gradually blended together. A rainbow is an example of a natural gradient. Questions about using gradients in graphic design may be more suited to graphicdesign.stackexchange.com. Questions about using gradients in programming and web development are off topic and should be asked on stackoverflow.com.
Gradients
A gradient is the outcome of blending multiple colors or channels gradually together. A rainbow is an example of a gradient.
Common types
The most common types of gradients are linear, radial, angle, reflected and diamond:

In digital media, the gradient is usually calculated using linear interpolation to gamma compressed RGB values.
Web 2.0 gradients
"Web 2.0" gradients are usually described as either:
- Sharp gradients, i.e. gradients that have a color stop between two colors without any blending (see e.g. the angle gradient at 12 o'clock under "Common types").
→ To make the object look glossy. - Subtle gradients, where colors either blend very slowly or where the blended colors are close together.
→ To give the object texture.
Some examples of gradients that could be described as "web 2.0 gradients":

Fade effect
Using transparency instead of a color gives the image/object a fading effect:

(Image by @Johannes)