An archetype is described as a recurring symbol, theme, character, or setting in multiple works. It's something that's appeared in literature so often that it's very recognizable. For example, an archetypical theme is "love conquers all." Where have we *not* seen that before? An archetypical plot structure is the infamous love triangle.
A motif, though, is a recurring contrast, structure, or literary device that comes back in different forms. In Lord of the Flies, the beasties are symbols which are used as motifs which represent fear, but it comes in different forms.