If I am not mistaken, one of the words in countable, while the other is uncountable. I'll leave you to figure out which is which. You need to pay attention to details like this.

"brush" is more of a collective term for a variety of plants that happen to be around. "shrub" is a more specific genre of plants, and usually more tidy, as they are more associated with gardening than wilderness.

Thank you Eliot, you are right! Shrub is a little tree planted in a yard, Brush is uncountable and maybe could be described as a group of shrubs, right? On the other hand, what about bush and shrub?, are they synonyms?