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
Trojan:W32/Skintrim is very similar to Adware:W32/Navipromo. Major differences between the two are:
Skintrim creates a mutex with the name of mymutsglwork
It injects code into explorer.exe, WLMail.exe, Winmail.exe, Outlook.exe and Thunderbird.exe
It hooks certain API such as 'send' of ws2_32.dll and 'OpenProcess' of kernel32.dll
It communicates with the updates.advert-network.com server to download a new component.