PsExec is a light-weight telnet-replacement that lets you execute processes on other systems, complete with full interactivity for console applications, without having to manually install client software.
PsExec is a Windows command line tool written by Mark Russinovich and available through Microsoft's Windows Sysinternals: http://technet.microsoft.com/en-us/sysinternals/default.
From Sysinternals:
Utilities like Telnet and remote control programs like Symantec's PC Anywhere let you execute programs on remote systems, but they can be a pain to set up and require that you install client software on the remote systems that you wish to access. PsExec is a light-weight telnet-replacement that lets you execute processes on other systems, complete with full interactivity for console applications, without having to manually install client software. PsExec's most powerful uses include launching interactive command-prompts on remote systems and remote-enabling tools like IpConfig that otherwise do not have the ability to show information about remote systems.
The 'PS' prefix has nothing to do with PowerShell - it appears because a previous tool by the same author used a 'PS' prefix (PsList, for listing information about processes). Various related tools are packages together in a suite called 'PsTools'.
