PowerShell is a very powerful command line scripting language that can be very helpful when working with SharePoint. There are a couple of references out on the internet but I would like to highlight www.powershell.nu which is a site run by a colleague and friend of mine Niklas Goude.
One his site he is constantly putting up new PowerShell examples to use when working with SharePoint.

You should also check out Mattias Fors Blog that has made a AD to SharePoint list synchronization with PowerShell. Very cool and will eventually be included in our Deployment Module I wrote about a couple of days ago.