Nearly all Trusted Sender entries you want to create will be email addresses. This means that mail from that address, and only that address, will bypass our spam checks, and go straight to your Inbox.
If you correspond frequently with many people in a small business or organization, you may wish to make a domain a trusted sender. A domain is the portion after the @ sign in an email address. This should be used with caution! Do not, for instance, make common ISPs like aol.com, gmail.com, etc. trusted domains, or you will find your account flooded with spam. Even making paypal.com a trusted domain means you will be subjected to many phishing attacks. Many users should never need to make a domain a trusted sender, and even users who do should probably have no more than 10 trusted domains.
Both server and network addresses are for IP addresses only. If you do not know what an IP address is, you don't ever want to use these options -- always use an email or domain address. If you do use them, please use them with extreme caution.
Server addresses will let you make a particular IP address a trusted sender -- all mail coming from that IP address will automatically be accepted.
Network addresses will let you make a particular subnet a trusted sender -- all mail from that network will be accepted.