An ERP is an event-related potential. Really, it is a time-locked EEG record of activity related to a stimulus presentation or some cognitive event. ERPs are used by
cognitive psychologists to determine what's going on in the brain during specific mental processing.
To record ERPs, EEG is recorded from the brain of a participant concurrently with the presentation of a stimulus. The records are averaged together, in the hopes that random electrical activity will average out, leaving only the activity related to the processing of the presented stimuli.