PSI specialize in knowledge, control, and information. Their operatives are all psi-positive, which make them far more dangerous than a street criminal because they steal things from one's own mind. Either they take the information from the victim without their knowledge, or, on occasion, they will mentally control the victim to do something they would otherwise not want to do.
It is rare to find a PSI operative in their outfit. Quite often they will mentally disguise themselves to be ordinary people doing seemingly ordinary things. They will rob people of precious information such as security codes, bank and credit card information, PIN numbers, network passwords, and social security numbers.
PSI agents will not work alone. They will always work in groups. One will do the "theft", another will keep the victim distracted and mentally confused, and possibly a third to serve as backup.
It is suspected that they operate through a company called The Mind, Inc. However this cannot be confirmed.
(Please see their Champions Online Wiki entry for more information.)
Agents of PSI are seen in "Future's Guardian" #5.