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/Monderd.gen is a Generic Detection for variants in the Trojan:W32/Vundo family.
About Generic Detections
Unlike more traditional detections (also known as signatures or single-file detections) a Generic Detection does not identify a unique or individual malicious program. Instead, a Generic Detection looks for broadly applicable code or behavior characteristics that indicate a file as potentially malicious, so that a single Generic Detection can efficiently identify dozens, or even hundreds of malware.
For more information about Generic Detections, please see Other:W32/Generic