Risk assessment is the process where you:
Identify hazards.
Analyze or evaluate the risk associated with that hazard.
Determine appropriate ways to eliminate or control the hazard.
In practical terms, a risk assessment is a thorough look at your workplace to identify those things, situations, processes, etc that may cause harm, particularly to people. After identification is made, you evaluate how likely and severe the risk is, and then decide what measures should be in place to effectively prevent or control the harm from happening.
For definitions and more information about what hazards and risks are, please see the OSH Answers document Hazard and Risk.