Exposing for the foreground will produce a washed-out, over-exposed sky while exposing for the sky will leave the foreground dark and under-exposed. Cokin graduated gray ND filters are part clear, part neutral density. They allow the transition to be blended into the scene, often imperceptibly.
A ND.6-to-clear, with a two stop differential, will generally compensate the average bright sky-to-foreground situation and is also the most popular density of the series.
121M - 4X Medium is the same gentle graduation as 121L but with twice in density. Requires 2 stops of compensation.