Cylindrical memory bank
The Next Doctor
An infostamp was a cylindrical memory bank. It projected the contained information like a film projector. (The Next Doctor)
They were used by the Time Lords to record data from within the Matrix, such as temporal anomalies. (Songs of Love)
The Cybermen also 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 by the Daleks while they and the Cybermen were in the Void.
A Cyberman was able to repair an infostamp by holding it with the cap at one end open. 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. (The Next Doctor)