(don't?) Try explaining it a little like this...
The following is often IMO statements. a few times things i don't really believe, but sounded cool when i wrote them...
Roleplaying games usually contain the following elements emphasized to varying degrees:
Social networking: hanging out with friends, talking, chatting (sipping a fine Amarone while discussing fine points of politics and philosophy before and after actual play) etc.
When participating in the game, players and gm accept an unspoken social contract to promote whatever elements of the game each other player considers fun and entertaining.
Friendly Competetion: While rpg's (very true for DnD) usually have game elements resembling those of boardgames, they aren't really (i think?) true games as defined by game theory. But there certainly can exist a sort of friendly rivalry and competitiveness similar to that observed in other social games (as monopoly or trivial pursuit) even though the true goal of play is actually to continue playing, as is the case with any other social game. Unless playing with a RBDM, everyone is a winner. In the special case of monopoly, the aim is to finish the agony of playing a stupid boring game as quickly as possible. Dice are often used to introduce an element of randomness, though comprehensive rulesets still promotes understanding of the rules guidelines and creativity in order to remain competitive.
Cooperative Interactive Storytelling: A dnd campaign is (usually) a series of linked adventures, (most often) structured as simple adventure/fantasy stories with a beginning, middle and an end. The plot, setting and non-player characters in the stories are invented and described by the gm, modified in response to the players describing their characters actions and reactions to the gms story.
Amateur thespians: GM and Players participate in the telling of a fantasy story, as actors and directors would do in a movie. In rpg's the equivalent of a script is only known beforehand to the GM. The "actors" get the opportunity to surprise the "director" with clever improvisation resulting in introduction of plot twists and witty dialogue. Player "acting" is always done by describing his characters actions, ranging from describing a characters actions during the "boardgame" to deep in-character conversations or deliviring of improvised monologues. Dice are often imployed to introduce an element of randomness in how the story unfolds.
When described like this, to an outsider, RPG's must be boring and thus very safe
Maybe downplay the sipping of expensive wines part a bit.
and sorry for my ramblings. I'll just go get some sleep shall i?