Trojans are malicious applications that pose themselves as legitimate software in order to trick users to install them. Once on the victim's machine, it may run any number of malicious process to steal vital information or inflict damage to other software.
W32/Bagle.dm is a trojan that disables security related processes and blocks access to security related sites.
Once installed, this trojan does DNS lookup for a list of domains to get their IP addresses. It then uses these IP addresses to block access to the domains.