If you crush something, you press on it hard such that it becomes smaller and damaged, or flattened and damaged. E.g. you can crush garlic by pressing it with a knife. E.g. in a scrapyard, old cars get crushed. To crush something = écraser quelque chose ?
“Crushing” is the present participle of the verb “crush”. So if I say “I am crushing some cashew nuts”, it means that I am doing it right now. Or if I say “The old cars are made smaller by crushing them”, it means that “crushing” is the method used to make them smaller.
There is also the noun “crush”. If you have a crush on someone, it means you fancy them.
“To crash” is a different verb to “to crush”. If a bird crashes into a window, it means that it hit the window and was hurt/damaged. Or a car could crash into a wall. Or I could crash into a wall (I would probably be in a car or on a bike). To crash = s’écraser ?
As a noun, a “crash” could be the event of a something crashing, or a very loud noise made when something falls or breaks, or a sudden fall in value (e.g. stock market crash).
If a computer crashes, it means it has stopped working (se planter ?).
I think those are the main uses of these words.