In order of lower to higher;
Crush = Really should just be called infatuation.
Like is = Well, you don't DISlike it/them, and sometimes makes you a little happy.
In-Love = Very strong feelings for another, but not to be confused with 'Love'. It's normally based on sexual feelings whether you believe so or not, it's a very powerful emotion and easily confused with 'Love', but it's also very very easy to fall 'out of love' with someone, (if they hurt you - let's say - in anyway).
Love = Yes, it's the ultimate feeling; You want to spend the rest of your days and would probably give your life for that person (though this is often true of being 'in-love' too, but often it's mistaken for what it is) and that's why it (the word) is so overused.
Think about it; You can LOVE your family, your girlfriend or boyfriend and your pets and especially your kids.
Notice how no one (usually anyway) is IN-love with their kids or family or pets?
That's because it IS based more on sexual mores than anything else. Though at least being 'in-love' with someone often leads to a more stable 'Love' situation.