PowerShell is a command-line and scripting language framework for task automation and configuration management. I didn't specify for Windows as, for a couple years now, PowerShell is cross-platform; however, it's a Microsoft product. These days it's built in to Windows, and despite its powerful potential for an attacker, it isn't going to be fully blocked. For the Windows pen tester of today, it's a comprehensive and powerful tool in your arsenal that just so happens to be installed on all of your victim PCs.