A vulnerability assessment is a process to identify and prioritize vulnerabilities present in a system.
Vulnerabilities can be a part of any system; however, as far as the cloud environment is concerned, vulnerability assessment is mostly done for servers and applications.
Vulnerability scanners are tools that will assess the applications and servers to search for any vulnerabilities and report to you in a nice little interface.
Apparently, vulnerability assessment is also one of the initial things that an attacker performs before he tries to break into your environment.
Let's look into a sample scan report by Nikto, which is basically a web server scanner:
In the following report, although there are no known vulnerabilities, there are suggestions related to adding X-XSS-Protection, X-Frame-Options, and X-Content-Type-Options header...