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.
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Rogue:W32/DotTorrent.A identifies a trojan which displays fake messages indicating an "Antipiracy foundation scanner" has discovered torrent files on the computer violating the copyright of the file's respective owners. This is an attempt to frighten the user into paying money to a (fictional) organization, ICPP, supposedly to pay for the copyright violation fine and for a 'legal license purchase'.
The trojan's extortion scheme qualifies this malicious program as a type of Ransomware.
DotTorrent's activities are more thoroughly discussed in the following F-Secure Labs blogpost:
On execution, the trojan displays a message box that purports to be a 'copyright violation alert':
The user is then directed to a website where payment may be made for the alleged violation.