Unlike the curl command line utility Invoke-WebRequest returns an object with various properties of which the content of the requested document is just one. You can get the content in a single statement by expanding the property like this:
Invoke-WebRequest 'http://www.example.org/' | Select-Object -Expand Content
or by getting the property value via dot-notation like this:
(Invoke-WebRequest 'http://www.example.org/').Content
Alternatively you could use the Windows port of curl:
& curl.exe 'http://www.example.org/'
Call the program with its extension to distinguish it from the alias curl for Invoke-WebRequest.