An infostamp was a cylindrical memory bank. It projected the contained information like a modern film projector.
The Cybermen used infostamps to transmit information to their databanks and inserted them into their chest cavities to learn particular subjects. Some of the data stored on these infostamps was stolen from the Daleks while the Cybermen and they were in the Void.
A Cyberman was apparently able to detect if an infostamp was corrupted simply by holding it with the cap at one end open. It was similarly able to repair the infostamp by doing much the same thing.
The core of an infostamp was a highly potent energy unit. Breaking the seal of the core released a dangerous energy beam. Thus, they could be used as an energy weapon which had devastating effects on Cybermen. However, if used unintentionally like this without knowledge of what they were, this could cause a backfire of information into the mind of another. Jackson Lake ended up thinking he was the Doctor. (TV: The Next Doctor)