I am not sure that there are one website that describes what can be done but the capabilites you get is Mobile Device Management where Window 8.1 is included in an BYOD scenario.
A good starting point would be here: http://technet.microsoft.com/en-us/library/jj884158.aspx
And then continue on, all setting that can be managed are listed here: http://technet.microsoft.com/en-us/library/dn376523.aspx there are also links to Applicaiton deployment, VPN profiles and much more.
Have a look at my guide for Mobile Device Management in SCCM 2012/Intune (it's almost finished). This will give you a lot of information.
No, once you integrate SCCM 2012 and Intune all management is carried out in SCCM.
You will not be able to see any devices in Intune (mobile or computers).
Computers will be managed via SCCM only. If you want to manage Internet Based computers you will have to implement SCCM IBCM (Internet Based Client Management). This is not the same as I
so they have DirSync and ADFS already. Is there anything we would need to do for Intune?
No, from a DirSync / ADFS point of view.
Depending on the size of the customer and the depth of management they require for their PCs, they could just use Intune stand alone without connecting to SCCM. If their main concern is laptop management with OS updates and software installs then Intune on its own would be reasonable and of course they would get the MDM capabilities as well.
Limitations of Windows Intune not connected to SCCM:
-No more than 7k users per tenant
-No "advanced" PC management features, like OS Distribution
Advantages are that since DirSync and ADFS are already set up, they can get up and running quickly.