PorterDuff.Mode is used to create a PorterDuffColorFilter. A color filter modifies the color of each pixel of a visual resource.

ColorFilter filter = new PorterDuffColorFilter(Color.BLUE, PorterDuff.Mode.SRC_IN);

The above filter will tint the non-transparent pixels to blue color.

The color filter can be applied to a Drawable:

drawable.setColorFilter(filter);

It can be applied to an ImageView:

imageView.setColorFilter(filter);

Also, it can be applied to a Paint, so that the color that is drawn using that paint, is modified by the filter:

paint.setColorFilter(filter);

An Xfermode (think "transfer" mode) works as a transfer step in drawing pipeline. When an Xfermode is applied to a Paint, the pixels drawn with the paint are combined with underlying pixels (already drawn) as per the mode:

paint.setColor(Color.BLUE);
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));

Now we have a blue tint paint. Any shape drawn will tint the already existing, non-transparent pixels blue in the area of the shape.

/**
 * Apply a radial mask (vignette, i.e. fading to black at the borders) to a bitmap
 * @param imageToApplyMaskTo Bitmap to modify
 */
public static void radialMask(final Bitmap imageToApplyMaskTo) {
    Canvas canvas = new Canvas(imageToApplyMaskTo);

    final float centerX = imageToApplyMaskTo.getWidth() * 0.5f;
    final float centerY = imageToApplyMaskTo.getHeight() * 0.5f;
    final float radius = imageToApplyMaskTo.getHeight() * 0.7f;

    RadialGradient gradient = new RadialGradient(centerX, centerY, radius,
            0x00000000, 0xFF000000, android.graphics.Shader.TileMode.CLAMP);

    Paint p = new Paint();
    p.setShader(gradient);
    p.setColor(0xFF000000);
    p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
    canvas.drawRect(0, 0, imageToApplyMaskTo.getWidth(), imageToApplyMaskTo.getHeight(), p);
}