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Fumaça vs. fumo

Word practice demo
Both fumaça and fumo refer to the gas produced by combustion, but the choice between them depends heavily on the variation of Portuguese being spoken. Generally, fumaça is the standard term for a visible cloud of smoke in Brazilian Portuguese, while fumo is the standard term used in European Portuguese.

Fumaça

A2
This is the most common word in Brazilian Portuguese to describe the physical, visible cloud of gas created when something burns. In European Portuguese, this word is also understood but is often reserved for a large, thick, or suffocating cloud of smoke.
Há muita fumaça saindo da janela da cozinha.
(There is a lot of smoke coming out of the kitchen window.)
A fumaça da fogueira irritou os meus olhos.
(The smoke from the bonfire irritated my eyes.)
O motor do carro quebrou e começou a soltar fumaça preta.
(The car engine broke and started releasing black smoke.)
Os nativos usavam sinais de fumaça para se comunicar.
(The natives used smoke signals to communicate.)

Fumo

A2
In European Portuguese (Portugal), this is the generic and standard word for smoke coming from a fire. However, in Brazilian Portuguese, fumo rarely means the gas cloud; instead, it usually refers to rope tobacco (a specific type of tobacco) or the act of smoking itself, appearing as the noun for smoke only in old literature or specific technical compounds.
O detetor de fumo disparou por causa das torradas queimadas.
(The smoke detector went off because of the burnt toast.)
Não consigo respirar com todo este fumo na sala.
(I cannot breathe with all this smoke in the room.)
Vimos uma coluna de fumo a subir da floresta.
(We saw a column of smoke rising from the forest.)
O fumo das fábricas escureceu o céu da cidade.
(The smoke from the factories darkened the city sky.)

Summary

The main difference is regional: use fumaça if you are in Brazil to describe the visible gas from a fire. Use fumo if you are in Portugal to describe the same thing. Be aware that using fumo in Brazil may be interpreted as referring to tobacco rather than the smoke cloud itself.