Viking ship tackling the wind
The civilian trade ship was called knarr.
The long ship was relatively shallow going (allowing for rapid deployment on sandy shores), sporting a rectangular sail and oars along almost its entire length. Oftenmost longships were clinker built, and waterproofed by moss drenched in tar.
Our best evidence for longship construction comes from ship-burials. It was common in Viking society for kings to be buried under a long burial mound in a ship with rich possessions. The Viking Oseberg ship burial in Norway and the Anglo Saxon longship of Sutton Hoo in England are both good examples.
Longships really were long, and narrow - quite extremely so. The largest discovered longship (at Roskilde harbor) is 35m in length, and the longship from Hedeby harbour has the largest length/width proportion: 11.4 to 1.