A. A game jam is when multiple people create games at the same time with a very short time limit. The games are all based on the same "theme",
B. You find out when the next one is and create a game in that time,
C. No idea
D.
SFML Game Jam Info And RulesYou may also want to join the
SFML Game Jam website as that is where the games are submitted and the themes are voted for, although I'm not sure how up-to-date it is at the moment.
EDIT: added link to jam website