The word fuego is probably one of the first words that Spanish language learners learn. 

It’s the most general word to refer to fire, as in the product of combustion.

It can be used when asking someone for a lighter (¿tienes fuego?), or the fire that burns on a bonfire or a campfire (el fuego de la hoguera), the flames of a fire (las llamas del fuego) and even in the sense of gunshots when someone shouts ‘hold your fire! (¡Alto el fuego!).

And it’s also the first word people will exclaim if a fire breaks out – ¡Fuego! (Fire!).

But when a fire is out of control, Spanish speakers rarely use the word fuego to describe this conflagration (yes, that’s a formal way of referring to an extensive fire in English). 

Instead they will call it un incendio (a fire) or el incendio (the fire). If it’s a wildfire or forest fire, they call it un incendio forestal.

That’s not to say you can’t use el fuego to refer to the fire in the general sense, but technically speaking if it’s a fire that’s broken out in a building or a forest fire that’s raging you should use the word incendio.  

There’s also the verb incendiar, to burn down or set fire to, in the active sense of someone choosing to burn something which sees the flames spread. You can also say prender fuego.

Or in the passive sense, as in a forest catching fire, incendiarse.

Examples:

Un incendio forestal en Barcelona ha arrasado miles de hectáreas de bosque.

A wildfire in Barcelona has destroyed thousands of hectares of forest. 

Los bomberos intentaron apagar el fuego en un edificio de la Gran Vía pero al final el incendio se cobró tres vidas.

The firefighters tried to extinguish a fire in a building on Gran Vía but in the end the blaze claimed three lives. 

Es un pirómano, ha incendiado un hermoso bosque porque le gusta ver cómo las cosas arden. 

He’s a pyromaniac, he set fire to a beautiful forest because he likes to see things burn.