This is a huge area of photography all by itself, with only partial overlap with more conventional genres. 'Deep sky' refers to (typically) objects beyond the solar system. These include stars, star clusters, galaxies and nebulae.
Many astro-imagers use DSLRs, usually converted to increase the red sensitivity through the removal of an internal filter (this does void the warranty, of course). This is because many sky objects such as nebulae radiate in this part of the visible spectrum and straight DSLRs have (deliberately) reduced sensitivity here.
Exposures are typically measured in minutes, rather than fractions of a second - and DSLRs are very noisy with exposures this long when at or near room temperature. A partial solution is to take many near-identical exposures and stack them in special software, coupled with the subtraction of dark frames (derived from shooting identical length exposures with the shutter closed), which between them removes much of the noise.
The other (and more expensive) solution is to use special cooled astro-cameras. These can cool the sensor by 30-40 degrees Celcius below ambient. This level of cooling, combined with stacking and dark-frame subtraction, can really produce some amazing images.
Almost all the images in my Astrophotography gallery were taken with modified 12-megapixel DSLRs