A caucus is a private party were meetings are held in the beginning of each year and they each party chooses their party leader. There is the majority party and its the party with the most members and there is also another party and that is the political party with the fewest members.
What is the session? Well the session is the meeting of congress and it starts in January and ends in November. There is one thing called the joint session is when the House and Senate meet together and talk.