Answers:
A very brief answer is that if you look over the case load of a court such as the Supreme Court you will see where laws passed by various legislatures and approved by the executive branch are up for review by the court. When the court strikes down such a law, it is checking the power of these two branches by not letting them use their power to pass unjust laws. I'm not completely sure of my memory, but if you check out the bus strike led by Martin Luther King in Montgomery Alabama, I think you will see an example where the Supreme Court knocked down a Montgomery law. This is probably one of the most famous examples the situation you are asking about.