"Correo electronico" is Spanish for electronic mail, more commonly referred to as e-mail. The literal translation is "mail electronic," in other words, mail or post sent via an electrical device.
Spanish is one of the five Romance languages which originated from Vulgar Latin. Vulgar Latin was a more colloquial Latin spoken by the people of the Roman Empire, as opposed to the Classical Latin used by writers in those times to produce literary works.
Spanish, and the other four Romance languages (French, Italian, Portuguese, and Romanian), almost always place adjectives after the noun which is being described, and this is the case with the mentioned expression. "Correo electronico" places the content word, the noun "correo," at the beginning of the phrase followed by an adjective that describes how the mail is sent, in this case by electronic means instead of physically.
Spanish expressions for other types of mail are "correo aereo," or what is referred to as airmail, "correo certificado," or registered mail, and the one most people hate to find in their mail boxes, "correo basura," or junk mail. "Correo urgente" translates literally as "mail urgent" or "urgent mail," but the best English translation for this expression is special delivery.