Windows RT is a touch-friendly, battery-sipping version of Windows for the ARM processor architecture. It's built on the same Windows "core" as Windows for x86/64 hardware, though the kernel was re-architected to run on ARM. It also has a great deal in common with Windows 8. Like Windows 8, Windows RT has the "Metro" GUI on top and the classic Windows desktop underneath. It can be driven with fingers alone or keyboard and mouse. It supports multiple users, so you can share your Windows RT tablet with others and each of you can have your own environment. Windows RT shares a new printer driver architecture with Windows 8, and it can access Windows HomeGroups to share printers and files. It can talk to external storage and other USB devices.
Like Windows 8, Windows RT runs both "Windows Store apps" (formerly known as Metro-style apps) and Windows desktop apps. However, the only Windows desktop apps that Windows RT will run are bundled editions of Microsoft Word, Excel, PowerPoint, OneNote, and Internet Explorer 10. The only applications that users can install themselves are Windows Store apps. This has big security benefits. Windows Store apps are Microsoft-curated and digitally signed, and they execute in a secure sandbox that controls their access to data and network resources outside the app container. If the app isn't installed through the Windows Store or signed by Microsoft, it doesn't run.
Of course the mobile-focused Windows RT has some important differences from Windows 8. It supports (or will support) cellular broadband radios and various hardware sensors (accelerometer, gyroscope, compass, GPS), though the Microsoft Surface RT has neither cellular radio nor GPS. And it takes a different approach to power management. Instead of the traditional Hibernate and Sleep power-saving modes, Windows RT introduces a new power mode, called Connected Standby, that combines a low-power state with always-on connectivity. Even when the screen darkens and applications are suspended, the device will continue to receive notifications and communications, as well as carry on other network-related tasks, in the background.
Another key difference from traditional Windows: Windows RT can't join an Active Directory domain. This might matter to you if you want to manage the configurations of Windows RT devices for business purposes. Instead of using Active Directory and Group Policy, you'll need to manage Windows RT via Exchange ActiveSync policies through Microsoft Exchange Server, System Center 2012, Windows Intune, or a third-party mobile device management solution. If your main concern is not configuration management, but keeping Windows RT devices updated, you can rest easy: All updates to Windows RT (firmware, drivers, apps, and so on) are delivered through the Windows Store or Windows Update.