as other suggested basically if you want to have Active Directory be available to your users outside the network you can implement:
VPN - vpn will give access to all shared devices, folders on your network and whatever rights you had set in AD it will be the same rights that will be applied when users are accessing VPN
If you want you can consider Terminal Services.
Normally we use remote desktop connection. Key in the IP fix address and the ports it will connect to the Window server. As for application we will create user with minimum privilege from the Domain controller.