Trojan horses are malware which are presented as legal software. They can’t spread by themselves.
Agent.REK drops and starts additional malware on the infected system.
W32/IRCBot.GNS is a Trojan horse with a backdoor inside.
Usually Trojan-Droppers contains different malicious files which are dropped to the infected system.
Tiny.E affects Explorer.exe and creates entry start-up point in the system.