I've heard some bad jokes in my time, and they fall into three groups.
1. Puns... terrible terrible puns;
Out in the open farming country, there lived a farmer with a hobby. He collected tractors. He had big ones and small ones, red ones and green ones, and everything in between. And of course, all this machinery took a lot of space, so he had a number of specially constructed buildings.
But slowly, over the years, he got bored of his collection, until one day he decided to get rid of the whole lot, and he set fire to them.
A neighboring farmer visited him, and found him standing at the entrance to one of the barns, sucking the smoke out of the building to keep the fire burning.
"What's going on?" asked the visitor.
"Well," he said, "I used to love these machines, but now I've become an ex-tractor fan."
2. Senseless and overused jokes;
A man walks into a bar... ouch! -or- Why did the chicken cross the road.
3. Jokes that are REALLY racist, or sexist.... I'm not going to share any here.