What are they?
Transform boundaries occur when plates slide next to each other, this can create faults which is when there is a crack in the Earth's crust.
This can create earthquakes and faults. The type of stress that is occurring is shear because the plates are sliding next to each other in opposite directions.
What has happened?
On April 18, 1906 the people of San Francisco were woken by a massive earthquake. It killed 3,000 people and left about 200,000 people homeless. This earthquake occurred because the Pacific plate is sliding against the North American plate.