Liefering is a district of Salzburg and can be found in the very north. It is very densely populated, and its almost 15.000 residents make it the second-biggest district of the city after the neighbouring Lehen in terms of population. Liefering is socially strongly divided into Alt-Liefering (the historic village) and various neighbourhoods in more central locations. The latter ones have a high percentage of immigrant residents and are generally considered to be a shit area. Alt-Liefering, on the other hand, is a middle-class neighbourhood that is getting increasingly expensive.
For centuries, Liefering was a village of farmers and fishermen. The core of the place developed around the Lieferinger Pfarrkirche, a rather large, Baroque church. Here you can also find very old farmhouses such as the Baldehof or Schlossbauerngut. The latter is referring to Schloss Schönleiten, now home to a private, Catholic school for boys.
The inner parts of Liefering (seen from Salzburg) developed very rapidly after WWII. A new church was built in Liefering-Lehenau in 1980, the architects in charge were Rüdiger Stelzer and Walter Hutter. A late 19th century chapel can be found in the Christian-Doppler-Klinik. The latter is a late 19th century hospital for mental and neurological illnesses. The exhibition centre of Salzburg can also be found in Liefering, conveniently located near the motorway.
Over-all, most of Liefering comprises of residential areas that will be of little to no interest for international tourists. An often heard local slogan of Liefering is "the village inside the city". This is stupid for two reasons: The only part of Liefering that looks like a village is beyond the motorway and far from the city; those parts that are actually closer to the city are among the most urban, shabby and least "villagy" parts of Salzburg. There are much more central neighbourhoods that have more of a village flair than Liefering.
Liefering on the German Wikipedia
City of Salzburg, Official Website