microsoft.powershell.utility.psm1

PowerShell script module Reference

A .psm1 file contains a PowerShell script module. The file groups related functions into one unit. The Import-Module command loads the module, and the exported functions become available in the session. A module manifest, a .psd1 file, frequently comes with the module. Modules are the standard method to distribute PowerShell code.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.