Get-SPFeature:  Retrieves the SharePoint features based on a given scope.
There are 4 levels of scope; retrieving the enabled Features at each level behaves differently at each scope
  1. Farm      2. WebApplication    3. Site (Site Collection)       4. Web (Site)
Retrieving the enabled Features at each level behaves differently at each scope
To view all the installed Feature definitions
            Get-SPFeature  (If scope is not provided, then all installed Features are returned)
To view all features for a Farm:
Get-SPFeature  -farm (Retrieves all the enabled Feature in the farm)
To view all features for a Web Application:
Get-SPFeature  -webapplication http://server
To view all features for a  Site:
Get-SPFeature  -site http://server/sites/test
To view all features for a Web:
Get-SPFeature  -web (Retrieves all the enabled Features in the Web)
To view all the features on the Farm grouped by scope in a table use:
                Get-SPFeature | Sort -Property DisplayName, Scope | FT -GroupBy Scope DisplayName (using select displayname, scope will remove the id column)
Enabling and Disabling Features
Enable-SPFeature -Identity "PremiumSite" -URL http://servername
Disable-SPFeature -Identity "PremiumSite " -URL http://servername

Installing and Uninstalling Features

Install-SPFeature -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\FEATURES\MyFeature\feature.xml"
UnInstall-SPFeature -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\FEATURES\MyFeature\feature.xml"