The longsword is a straight, two edged sword with a hilt long enough to fit a second hand if necessary. The guard is tyically a simple cross but may have elaborate defences. It is only used as a slashing weapon and thus may be fullered, but would not have a spine or diamond sectioned blade.
As with many weapons in DnD it is a poor representative or misnamed version of its historical cousin. The historical arming sword closest fits the description of the DnD longsword. The historical longsword best fits the description of a bastard sword.