Why are some computer viruses called Trojan horses?

A trojan horse is a type of malware that can deal damage to your data and your computer, and not really a "virus". Trojan horses get their name from the works of Homer that describes of the military strategy used against the Trojans in the Trojan War. The Greeks used a wooden horse filled with a small contingency of troops inside as a "gift". Once inside, the hidden troops will infiltrate and attack from the inside.