PowerShell modules are like adding a plugin to your current PowerShell CMDLet library. Modules can be created in PowerShell and even shared among different people. Just be careful what you download or use from someone else to assure you know what the results will be. A perfect example would be a download of PowerCLI which is a module for VMware. With the module installed on my machines and the module imported into PowerShell I can get information on the entire virtual environment and also manage the virtual environment.
Another module that I frequently use is the Active Directory module to allow querying Active Directory for user and computer objects and obtaining information. I also can set attributes on these objects as well. I have even created a user creation tool to make sure that all users created have specific group membership and proper fields filled in.
Active Directory Module