RSAT is a set of components for managing a server locally or remotely. When you install a role or feature on a server through Server Manager, it will suggest that you install the Management Tools, or you can install it from Server Manager later. From PowerShell, you can include the -IncludeManagementTools option on the installation command, or use the Install -WindowsFeature command to install the Management Tools later.

But if you're running Hyper-V Server only, you don't have this option, as Hyper-V Server doesn't have Server Manager or MMC (Hyper-V Manager). In this case, you can install RSAT, Hyper-V Manager, and Hyper-V module for PowerShell for Windows 8.1 and manage your servers remotely.

RSAT is not native to Windows Client; you can download it from http://www.microsoft.com/en-us/download/details.aspx?id=39296.

RSAT is a standalone update for Windows 8.1, and, after installing it, you have to enable the consoles...