Getting back to the topic, I would say that people who play music are musicians. People who perform music are musicians and performers. This is true for all genres of music. Within folk music, it is much more common for non-musicians to take part in the music making, or rather I should say that a greater percentage of the people within the genre are musicians (in that they play music) than with other genres of music. This is great, but it doesn't mean that a performer of folk music isn't also a folk musician. They're just usually technically better at it and are willing to get up on stage and offer performances of it. The thread title and many of the posts seem to assume it's an either/or question. I strongly disagree.

As for what defines a performance of folk music, that is entirely subjective. The only thing we can say about it is whether or not we like patter between the songs, whether or not we like highly arranged music or simple statements of the tune or song, or if we like it when performers change the songs to suit one's fancy or ones idea of what the audience might like. None of this has anything to do with whether or not a someone is a folk musician or a performer. It's just about our personal tastes. It seems obvious that anyone who plays music for other people to listen to is a performer. How they conduct themselves within that is beside the point.