As risk register is a tool in the form or spread sheet, application or database that you can use during risk assessments for risk identification. It allows the person conducting the risk assessment to log the threat, asset and impact and give some idea of the probability of the threat. It is used in DIARMF, DIACAP, project management and other risk management processes to help decision makers.
Its also known as a risk log.
Here is a risk register template I got from British Colombia gov site http://www.gov.bc.ca/fin/):
Here is another risk register template from the Israel Institute of Technology(webcourse.cs.technion.ac.il):