The term "X-Rays" refers to light with wavelengths shorter than about 10 nanometers. (That's only 10 billionths of a meter! Compare this to the wavelengths of radio waves.) The picture above was taken with a camera that sees light with wavelengths between about 0.3 and 4.5 nanometers, the so-called "Soft X-rays." The X-rays we see all come from the corona, the outermost and hottest visible layer of the Sun's atmosphere.
Not all the corona emits the same amount of X-rays. We often see structures called "loops" and "arches" and "streamers". Some of these are found around active regions, while some are occasionally found in more quiet parts of the corona. Movies made from X-ray pictures show that the corona is a very stormy place, constantly changing and erupting. Look at the Sun again a few days from now to see how it has changed. In these X-ray images you can sometimes see the "coronal holes," regions of low brightness at the north and south poles. The coronal holes are also sometimes seen in the Extreme Ultraviolet images.
This picture of the Sun comes from the Soft X-Ray Telescope on the Yohkoh spacecraft, orbiting about 630 kilometers above the Earth. NOTE: The Yohkoh solar observatory is currently offline. The image above was made in December, 2001.