Abell 2218 is a massive cluster. So massive that it bends gravity around it to such an extent, light passing through that region of space gets distorted in the same way as light passing through the base of a wine glass does – the closer to the centre the light is, the greater the push towards the edge. In the Hubble images, that use near infrared and visible light, the result is a series of galaxies and thin curves that are the images of more distant galaxies magnified by this gravitational lens. In the new Herschel images, the result is an image of cold gas and dust from just 2.6 billion years after the Big Bang.
Gravitational lensing was a key prediction of General Relativity that was put to the test by Kendal born Arthur Eddington in the 1919 Principe Eclipse trip. He took images of the Sun in front of the bright Hyades cluster and measured the deviation in the position of the stars due to the presence of the mass of the Sun. Since that time, it has become a valuable probe of the more distant reaches of the universe, allowing us to glimpse stars and gas that otherwise would simply be too distant to notice as well. It also provides a way of measuring the mass of an object, as this is the primary source of the strength of the lensing effect. Most recently, it has been used to detect extrasolar planets passing between ourselves and other stars, most notably in the Andromeda galaxy. This creates an increase in brightness, in opposition to the small decreases those measuring transits of exoplanets between us and their host stars look for.
More interesting physics inherent in the images include the resolution of the two telescopes. Hubble has a crystal clear, high resolution image, yet Herschel is the largest telescope ever flown and has in comparison fuzzy blobs. The reason is the increase in resolution provided by the increase in the diameter of the mirror is more than compensated for by the increase in the wavelength of light used. The process of diffraction simply works against longer wavelengths, producing terrible results in radio wavelengths that require the use of massive dishes or even multiple separated telescopes added together to get much in the way of resolution.