Select an object or group (or target a layer in the Layers panel).

Choose Effect > Stylize > Inner Glow or Effect > Stylize > Outer Glow.

Click the color preview square next to the blending mode menu to specify a color for the glow.

Set additional options, and click OK:

Mode

Specifies a blending mode for the glow.

Opacity

Specifies the percentage of opacity you want for the glow.

Blur

Specifies the distance from the center or edge of the selection where you want any blurring to occur.

Center (Inner Glow only)

Applies a glow that emanates from the center of the selection.

Edge (Inner Glow only)

Applies a glow that emanates from the inside edges of the selection.

When you expand an object that uses an inner-glow effect, the inner glow reveals itself as an opacity mask If you expand an object that uses an outer glow, the outer glow becomes a transparent raster object.

Select the object or group (or target a layer in the Layers panel).

Choose Effect > Stylize > Feather.

Set the distance over which the object fades from opaque to transparent, and click OK.

Original object selected (left) and using feather effect (right)