Vulnerabilities are flaws that exist in various programs and that allow someone to make that piece of software do something its not supposed to do such as downloading and running malware. The majority of computers get infected from visiting a specially crafted webpage that exploits one or multiple software vulnerabilities. The bad guys use those newly discovered flaws in exploit kits (leveraging the hard work of others by simply copying and pasting what others wrote or adapting proof of concepts) to exploit the flaws in software.”]
Source: https://blog.malwarebytes.com/threat-analysis/2013/01/web-exploits-bright-future/