1) Fanon is something widely treated as true within a given fandom, usually (though not always) implied by the original canon. It's a prevailing factor that influences many people's experience of the material. There can be countervailing interpretations, but most folks in a fandom will be familiar with its major fanon points.
2) Headcanon is your individual interpretation of something. It does not influence anyone else, unless your rendition of it is so compelling that it becomes part of fanon -- which is actually one of the main sources for fanon.
One of the coolest compliments I've ever gotten on a comment I made to someone's post was, "New headcanon: accepted."