Sigurd is the Norse name for a hero who also survives under a second identity, Siegfried, in the Middle High German Nibelungenlied, and the two are best understood as regional descendants of one shared Germanic legendary tradition that split apart as it spread and was written down centuries apart in different languages.
The Old Norse versions, preserved in the Poetic Edda and the prose Volsunga Saga, and the German version differ on real points: the order of events, the motives given to the main characters, and especially the treatment of Brynhild, whose role and backstory are handled quite differently between the two traditions. Comparative philologists have used exactly these divergences to reconstruct something of the shared older legend behind both, on the reasoning that where the Norse and German versions agree, the material is likely to be old, and where they diverge sharply, one or both traditions have reshaped it to fit local taste and later literary convention.
The dragon Fafnir, the hoard, and the general shape of a doomed hero entangled with two women and a family feud recur in both, which is taken as evidence of a common source, while the specific texture of each surviving version reflects centuries of separate transmission before either was written down in the form now read.