This definition explains the meaning of trojan horse. Trojan viruses are so named after the huge wooden horse that was packed with Greek soldiers and given as a gift to the people of Troy, leading to the downfall of the city. In the same way, an innocuous email can lead to the downfall of your networks if you click on it.
Trojan viruses are disguise masters. You may come across software that seems useful via email or online, but does huge damage if you download it to your computer. People are tricked into thinking they are clicking on legitimate files from a legitimate source. Trojans may contain the usual silly pranks or do real damage by destroying information, creating backdoors for hackers or compromising your personal data. They don’t reproduce or self replicate but can do great damage and cause huge frustration.