Combustible vs. carburante
The Spanish words 'combustible' and 'carburante' both refer to 'fuel'. However, they are used in slightly different contexts.
Combustible
'Combustible' corresponds to the English 'fuel'. It is a general term that refers to any material that can be made to react so as to release energy from it.
El petróleo es un tipo de combustible fósil.
(Oil is a type of fossil fuel.)
Para la barbacoa necesitaremos combustible para encender el fuego.
(For the barbecue, we will need fuel to start the fire.)
Este motor puede funcionar con varios tipos de combustible.
(This engine can work with various types of fuel.)
Carburante
'Carburante', similar to 'fuel' in English, typically refers to a type of fuel specifically meant for running engines, like in cars or airplanes.
Necesito poner carburante en el coche antes del viaje.
(I need to put fuel in the car before the trip.)
El avión necesita más carburante para completar el vuelo.
(The airplane needs more fuel to complete the flight.)
Los precios de los carburantes han subido de nuevo.
(Fuel prices have gone up again.)
Summary
'Combustible' and 'carburante' are both Spanish terms that translate into 'fuel'. The term 'combustible' refers broadly to any material that releases energy whereas 'carburante', whilst also meaning fuel, tends to be used in specific reference to engine fuel such as gasoline or diesel.