Understanding the weaknesses and vulnerabilities in a system or network is a huge step toward correcting the vulnerability or putting in appropriate countermeasures to mitigate threats against those vulnerabilities. Potential network vulnerabilities abound, with many resulting from one or more of the following:
Policy flaws
Design errors
Protocol weaknesses
Misconfiguration
Software vulnerabilities
Human factors
Malicious software
Hardware vulnerabilities
Physical access to network resources
Cisco and others have created databases that categorize threats in the public domain. The Common Vulnerabilities and Exposures (CVE) is a dictionary of publicly known security vulnerabilities and exposures. A quick search using your favorite search engine will lead you to the website. There is also a National Vulnerability Database (NVD), which is a repository of standards-based vulnerability information; you can do a quick search for it, too. (URLs change over time, so it is better to advise you to just do a quick search and click any links that interest you.)