Casco vs. yelmo
The Spanish language, like all languages, may have multiple words for the same object. Such is the case with 'casco' and 'yelmo', both of which can be translated to English as 'helmet'. However, despite this shared translation, they are often used in differing contexts and can convey distinct images or ideas.
Casco
'Casco' can be generally translated as 'helmet', referring broadly to any kind of protective headgear. Besides that, 'casco' is frequently used to refer to the shell of an animal or the hull of a ship. The specific context is key in determining its intended meaning.
El casco de la moto es obligatorio en muchos países.
(The motorcycle helmet is mandatory in many countries.)
La tortuga tiene un fuerte casco que la protege.
(The turtle has a strong shell that protects it.)
El casco del barco está hecho de acero.
(The hull of the ship is made of steel.)
Yelmo
'Yelmo' also means 'helmet', but is typically used more specifically to refer to the helmets worn by knights or soldiers in historical or fantasy contexts. It imparts an image of a medieval, often ornate helmet, possibly with features such as a visor or crest.
El caballero llevaba un yelmo resplandeciente en la batalla.
(The knight wore a shining helm in the battle.)
El yelmo de Darth Vader es icónico en el cine de ciencia ficción.
('Darth Vader's helmet is iconic in science fiction cinema.)
Summary
'Casco' and 'yelmo' both translate into English as 'helmet', yet they're not completely interchangeable. The term 'casco' broadly refers to any protective headgear (or shell/hull), whereas 'yelmo' evokes an image of knightly or military helmets from historical eras or works of fantasy.