A graph that breaks a big probability model into variables and small factor pieces.
A factor graph is like small group chats for one big family. Each chat holds one matter and only the people it touches. News hops from chat to chat until everyone agrees.
You meet it in probability inference and SLAM. It draws who depends on whom, then passes messages along the links.
PGM
A factor graph is a variable-and-factor form of a PGM.
BP
BP often sends local messages back and forth on a factor graph.
Bayesian Network
A Bayesian Network can become a factor graph for easier inference.
SLAM
SLAM often uses factor graphs to combine sensor clues.