A collision is where two things physically crash into each other. So a car can have a collision with another car. Or two people who aren't looking can have a collision with each other. A clash is where two things are at odds or against each other. They don't go together or they have an issue with each other. Two groups of people can clash - one is for more taxes and one is for less - they are against or opposite each other. Two colors can clash - you don't wear green pants and a pink shirt - those two colors clash. Or you can have two football teams that clash when they play a game - they are against each other. Of course, it can be more than two things, those are just examples.