Jamieson, Fausset and Brown on Mark 16:5

5. And entering into the sepulchre, they saw a young man--In Mt 28:2 he is called "the angel of the Lord"; but here he is described as he appeared to the eye, in the bloom of a life that knows no decay. In Matthew he is represented as sitting on the stone outside the sepulchre; but since even there he says, "Come, see the place where the Lord lay" (Mt 28:6), he seems, as Alford says, to have gone in with them from without; only awaiting their arrival to accompany them into the hallowed spot, and instruct them about it.

sitting on the right side--having respect to the position in which His Lord had lain there. This trait is peculiar to Mark; but compare Lu 1:11.

clothed in a long white garment--On its length, see Is 6:1; and on its whiteness, see on Mt 28:3.

and they were affrighted.