punch lines are usually the final part of a joke or u can call it the the climactic phrase or statement of a joke,producing a sudden humorous effect.
E.g;
A man walks into a bar with a duck under his arm.
The bartender asks: "Say, where did you find the pig?"
"It's not a pig, it's a duck," the man answers.
TO WHICH THE BARMAN REPLIES: ''I WAS TALKING TO THE DUCK''
(punch lines are in capital letters)