A trojan propagates by deception, unlike most other viruses or worms. The payload (or contents) of the program could consist of a root kit or back door, a memory resident virus, or any sort of direct attack on the victim user's programs or operating system. They could be very destructive, or simply designed to try to gather information such as e-mail addresses or usernames and passwords. Spyware is often just added to a program, so the victim does get what they expected--but with spyware included! As you can imagine, the trojan malware's content might be any sort of programming the trojan's creator might want to include!
Often a trojan will appear to be something that an inexperienced or careless user might want. Sometimes trojans send themselves out to e-mail contacts from an infected user, so that they appear to be sent by a friend or family member. They will have deceptive names, like windowsupdate.exe, or patch.exe, and so forth. Names like this that suggest that they are updates, patches, or malware checking or removal programs are common. Since the spread of media files, some trojans are able to trick users by appearing to be humorous or interesting videos. Often users have file extensions hidden, so a trojan might even appear to be something other than an executable file.
