Aliases that the system is expected to know are also kept here in the aliases file. Other files that are kept in this directory include the hosts file. The host file keeps a permanent list of IP addresses for hosts the computer might need to reach. It's not used much anymore, but was once a standard feature of UNIX computers in the days before the DNS system was adopted. Two special hosts files will allow you to accept or deny connections from specified Internet hosts.
Shared libraries are software routines that are used by more than one part of the operating system.
In this case, these routines are kept in a common place and are accessed as needed. You might also get error messages saying that a certain application cannot be run because a specific library is missing. UNIX applications normally check the lib directories during installation to see if the libraries needed by the application are already installed. If they are, the already- installed library is not usually overwritten. The sbin directory contains binaries, just like the bin directory does, but these binaries are related to the system administration functions of a UNIX computer.