What is vulnerability and how to prevent from occurin vulnerability in system.?

Vulnerabilities are what information security and information assurance professionals seek to reduce. 

Cutting down vulnerabilities provides fewer options for malicious users to gain access to secure information. 
Computer users and network personnel can protect computer systems from vulnerabilities by keeping software security patches up to date. 
These patches can remedy flaws or security holes that were found in the initial release. 
Computer and network personnel should also stay informed about current vulnerabilities in the software they use and seek out ways to protect against them.