A sniffer or a spoofer is usually a standalone program to
intercept and analyse certain data. For example a sniffer can
intercept and analyse network traffic and catch certain data, for
example passwords. Trojans sometimes use sniffing capabilities to
steal passwords and user information from infected computers.
There also exist a lot of commercial and free sniffers. They can
be used to analyse network traffic for perfomance, security
issues and faults.