Ônibus vs. autocarro
The Portuguese words ônibus and autocarro both translate to bus in English. The primary difference between them is geographical, as one is used in Brazil and the other in Portugal.
Ônibus
A1This is the standard word for bus in Brazilian Portuguese, referring to a large motor vehicle used for public transportation along a fixed route.
Eu pego o ônibus para ir ao trabalho todos os dias.
(I take the bus to go to work every day.)
Onde fica o ponto de ônibus mais próximo?
(Where is the nearest bus stop?)
Nós vamos viajar de ônibus para a praia no fim de semana.
(We are going to travel by bus to the beach on the weekend.)
Aquele ônibus de dois andares é para passeios turísticos.
(That double-decker bus is for tourist tours.)
Corri muito, mas infelizmente perdi o ônibus.
(I ran a lot, but unfortunately I missed the bus.)
Autocarro
A1This is the standard word for bus in European Portuguese, serving the exact same function as ônibus does in Brazil.
Eu apanho o autocarro para ir para o trabalho todos os dias.
(I take the bus to go to work every day.)
Onde fica a paragem de autocarro mais próxima?
(Where is the nearest bus stop?)
Vamos comprar os bilhetes de autocarro para a viagem ao Porto.
(We are going to buy the bus tickets for the trip to Porto.)
O autocarro número 15 vai para o centro da cidade.
(Bus number 15 goes to the city center.)
Este autocarro tem ar condicionado e acesso à internet.
(This bus has air conditioning and internet access.)
Summary
In essence, the difference is purely regional. If you are in Brazil, you will use and hear the word ônibus. If you are in Portugal or other Lusophone countries in Europe and Africa, you will use and hear autocarro. They are perfect synonyms, but their usage clearly signals whether you are speaking Brazilian or European Portuguese.