For instance securing your corporate accounting department or your network from your subsidiary's network. Many client applications exist to make the process easy. Typically members of a group will share common characteristics, such as being in the same department of a company. If a rule applies to a group then the rule then applies to all members of that group. You might have 'Administrators' , 'Users' and 'Dial-in' groups, each allowing different access rights.
Users can belong to one, several, all or no groups.
This stores some info about where certain machines are. WinGates' WWW proxy allows HTTP access to LAN users. This is secure http. Netscape and other browsers have built in encryption, to make data exchange more secure. This is commonly used for Online purchasing, especially where Credit cards are involved.