Three types of security problems are faced by a Network.
1)Physical: Unauthorized access and tampering of your files.Hackers do this by guessing and making permutation and combination of passwords.This is akin to creating duplicate key and doing theft .
2)Software:Loopholes or weakness in the programs written for the network.This can be compared to the lacunae in the government or any laws:people do mischief by using these gaps in the system.
3)Improper set up: the hardware and software are assembled deviating from the logical design. You can compare the fixing of improper wiring in out house leading to the short-circuiting.
1) Authorization by the set of ids or user names provided or allowed by the Service provider.
2) Authentication is the password or other codes given to the user for requesting access.
3)Encryption: Data is ciphered (the meaningless set of charter's) and deciphered (into the meaningful sets) at the service user and provider ends.
4)Biometric system:The most secured method compared to other methods. The unique nature of a person's body such as finger-print,retinal pattern,etc..is used to provide the identity.The advantage here is this is useful for Blind and Illiterate people.
5)Firewall: This is possible in both hardware and software or a mix of them.Only when a request meets the standards of the network , the access is allowed. Even emails will be filtered with this method.
6)Multi-level security: This is the latest where an user has to pass the two or more levels of authorization/authentication. This is most widely used in Banking circles for their online banking facilities.