Network security is the study and practice of making a computer network secure from malicious users. This can be a complex task that people are frequently paid large sums of money to perform. It involves configuring routers and unix boxes, as well as performing social engineering to ensure that legitimate users follow certain guidelines (such as keeping their password secret).
The only way to make a network completely secure is to disconnect it from all other networks, particularly the Internet. Some would say that even this is not completely secure, since malicious users with the right spy equipment could still compromise the network.