The Zeus Crimeware Toolkit is a favorite among Malicious hackers due to its flexibility, advanced features and easy usability. In this video created by Symantec we see a demo of how to setup Zeus by creating a unique malicious file and using it to infect victims. Once a victim is infected, it is under full control of the hacker. The hacker now can execute arbitrary commands, upload and run executables, monitor keystrokes and even watch the remote user using remote desktop like features.
The video is very detailed and I would recommend watching it. A detailed blog post by Symantec's Peter Coogan is
available here.