Snorri Sturluson based his work on an earlier Ynglingatal which is attributed to the Norwegian 9th century skald Þjóðólfr of Hvinir, and which also appears in Historia Norwegiae. It tells the most ancient part of the story of the House of Ynglings (Scylfings in Beowulf). Snorri described the descent of the kings of Norway from this royal house of Sweden.
Ynglinga saga is the first part of Snorri's history of the ancient Norse kings, the Heimskringla. Snorri's work covers the history of the Norwegian kings from the mythical prehistoric age until the year 1177, with the death of the pretender Eystein Meyla. Interwoven in this narrative are a number of references to important historical events.
The saga deals with the arrival of the Norse gods to Scandinavia and how Freyr founded the Swedish Yngling dynasty at Uppsala. Then the saga follows the line of Swedish kings until Ingjald (Ingjald illråde), after which the descendants settled in Norway and became the ancestors of the Norwegian King Harald Fairhair.
Read more about Ynglinga Saga: Sources