I believe the most simple way to explain that difference is, American English and British English is different as Brazilian Portuguese and European Portuguese, both have different accent, expressions and homophones. You can communicate but we brazilians usually can't understand very well the portuguese accent because all of their sentences usually sound short, and we articulate more the words, we use more our tongue and open more our mouth, so the sound is completely different. We take some time to start undestanding clearly a portuguese person, but they can undertand us easily.