Bocina vs. claxon
The Spanish words 'bocina' and 'claxon' both refer to the horn of a car. While they are used interchangeably in many contexts, there are subtle nuances that differentiate them.
Bocina
In most Spanish-speaking regions, 'bocina' is a general term for any kind of horn or loudspeaker. It could be a car horn, a loudspeaker at a concert, or even a megaphone.
La bocina del coche es muy fuerte.
(The car's horn is very loud.)
Utilicé una bocina para hacer el anuncio en la plaza.
(I used a loudspeaker to make the announcement in the square.)
Claxon
'Claxon' is derived from the English term 'klaxon', which specifically refers to an electric horn or warning device, such as those found on motor vehicles. In Spain and some Latin American countries such as Mexico, 'claxon' is often used specifically to refer to a car's horn.
El conductor tocó el claxon para advertir al peatón.
(The driver honked the claxon to warn the pedestrian.)
El claxon de ese camión es realmente estridente.
(That truck's claxon is really shrill.)
Summary
'Bocina' and 'claxon' are two terms in Spanish that can refer to a car's horn among other things. Generally, 'bocina' can refer to any type of horn or loudspeaker, while 'claxon', borrowing from the English term 'klaxon', is more specifically associated with electric horns or alarm signals usually seen in vehicles.