I'm in the midst of ordering a USB wifi-dongle for my Raspberry Pi. I have Raspbian installed (used NOOBS) on it, and my question is: If the specification of the wifi-dongle says it's Linux compatible, is it also Raspbian (Raspberry pi) compatible?
Raspbian is Debian Linux. If the USB WiFi dongle claims to be supported in current Linux kernels, then you should be fine. But if it includes a Linux driver, then you may be in trouble.
The reason for this is that vendors that provide Linux drivers are often doing so with opaque binary blobs compiled for x86 CPUs. The Pi boards are all based on ARM CPUs, so a binary blob targeting x86 won't work.
I'm using this Kootek WiFi adapter on my 2B, and it worked out of the box with no trouble.
I think that you are trying to determine whether the USB device is compatible with your stack: chipset (ARM) + OS (Debian). If that is the case, then it the question is whether a device driver is available for the stack.
Usually the hardware is implicit / assumed to be PC hardware (Intel / AMD CPU). If you are buying a USB wifi adapter for your rPi, stick to what others are using in the forums or Google the device to see if other are using it with rPi.