This is a close-up picture of the sun. To give some size comparison, each little popcorn-looking bright granule region is around 700 km wide. So (by rough counting) this picture is about three Earth-diameters wide. This patchwork of comparatively small bubbles is called granulation, and is caused by the convection zone of the sun bumping up against the surface. Each granule lasts for a pretty short amount of time, only five or ten minutes. I don't know how "correct" the comparison to a pot of boiling water is, but that's what this image brings to mind.

The big dark blob in the upper left is a sunspot. It's darker because it's colder than the rest of the surface of the sun – about 4000 K instead of 6000 K. The dark center of the sunspot is called the umbra (Latin for "shadow"), and the slightly lighter transition region around the center is called the penumbra (Latin for "almost shadow"). Sunspots are highly magnetic, and you can see the magnetic field lines in the filamentary structure of the penumbra.

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