All movies are entertainment, but only good movies are entertaining
The best distinction is the one represented in Collin's English Dictionary:
entertainment (ˌɛntəˈteɪnmənt) n.
1. the act or art of entertaining or state of being entertained
2. an act, production, etc, that entertains; diversion; amusement
versus entertaining (ˌɛntəˈteɪnɪŋ) adj.
1. serving to entertain or give pleasure; diverting; amusing