I found that a sledghammer works best though. Won't miss any trojans at all!

But yeah, it's liks tshdos said 123yebo. Trojans are files that appear to be safe to run, but in fact are not.

For example, there was a trojan going around that appeared to be a SFX file (self extracting compressed file) - ones you might download to install a seemingly legit program. When run, it'd show the ZIP file was corrupt and couldn't be extracted. But at that point, your system was actually infected with a trojan. This allows the Trojan to be fairly large (1 MB+) and thus usually a bit more sophisticated (but not always).