Is the Surface more like an Apple device, where you can't install any applications not found in the app store, or is it more like an Android , where you can install whatever third party apps you want?
That would be more like an Apple device.
Generally speaking, only apps from the store can be installed on the Win RT version of the Surface. The Surface Pro, which will feature Windows 8 (not RT) but isn't out yet, will be able to install applications that also run on Windows 7.
There will be, however, a way for organizations to deploy applications to devices associated with them (through Active Directory), without publishing on the Windows Store. An April 2012 post on the Building Windows 8 described this.
According to this link only Windows Store apps may be installed in Windows RT
Third parties may develop apps for Windows RT but they must be distributed via the Windows Store. Unlike Android which allows installation of APK from unknown sources, as of now, apps can be installed in Windows RT from Windows Store only.