From helpnetsecurity.com
A vulnerability assessment is a methodical examination of network infrastructure, computer systems, and software with the goal of identifying and addressing known security flaws. Once the vulnerabilities are pinpointed, they are classified based on how critical it is to fix/mitigate them sooner rather than later. Usually, the vulnerability scanning tool also provides instructions on how to remediate or mitigate the discovered flaws.
Security teams can use the findings of a vulnerability assessment to better understand the security posture of their network and put protective measures in place.
All the open-source vulnerability assessment tools listed below can be downloaded and used for free.