A trojan, or trojan horse, is a seemingly legitimate program which secretly performs other, usually malicious, functions. It is usually user-initiated and does not replicate.
Additional Details
This is the Trojan:W32/Vundo family description.
Vundo variants are components of an adware program that downloads and displays pop-up advertisements.
The adware is normally downloaded by users from the websites; these malicious sites are normally promoted by spam e-mails.