Attackers use Zeus, Spybot or other malware to push code onto the victims' computers and harvest banking information, which is then sent to seemingly legitimate Chinese business accounts. According to the FBI:
The unauthorized wire transfers range from $50,000 to $985,000. In most cases, they tend to be above $900,000, but the malicious actors have been more successful in receiving the funds when the unauthorized wire transfers were under $500,000.
The attacks are highly organized. Even though the money is sent to China, it is not known who is behind the attacks, or even if the stolen money stays in China.