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
A2Casco 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
B2Yelmo 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.