Pathlines: It is the path traced by a fluid particle over a given period of time.For unsteady flow,path lines for different fluid particles passing through the same point are not same.

Streamlines: Streamlines represents the curve where tangent at any point on the curve is in the direction of velocity vector, at that point.For an unsteady flow the streamline patterns changes at different times.

Streaklines: It is the locus of the fluid elements that have passed through a particular point over a period of time.

Pathlines, streamlines and streaklines are all same curves for a steady flow. For unsteady flow they are different.