Are 'Apple/Android Specific' USB charging ports really different? I have a portable battery with two USB ports. One is labelled Android and one is labelled Apple. Apparently each has charging circuits optimized for that particular type of device. This sounds unlikely to me (both ports appear to charge my Android device just fine), but I don't know much about it, especially regarding Apple devices. Is there any truth behind this? What is the difference between the ports and can I charge any device using either of the ports? I've also seen tablet and phone specific charging ports. Do they simply differ by maximum output current?
There are two USB charging standards; the official one and the Apple one. You can get a copy of the offical one from usb.org. How charging works depends on whether you are connected to a computer, USB hub or wall socket. For the sake of this discussion I'll just talk about wall sockets, what the USB spec refers to as a Dedicated Charging Port (DCP). Per the USB2.0 spec, a DCP shall short the D+ line to the D- line and supply 0.5 to 5.0A at 4.75V - 5.25V. However, a device being charged cannot draw more than 1.5A. (http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip)

Apple tends to do things its own way and the charger identifies itself to the device by the voltage it applies to the D+ and D- lines. A little write up is here https://learn.adafruit.com/minty-boost/icharging. An anecdotal story; I used to work for Nokia so had Nokia phones. I had a car charger that wouldn't charge the phone very fast but it was iPhone compatible. I pulled it apart and shorted the D+ and D- pins so to matched the USB spec and it then charged the phone properly.

One more thing to point out; it is the battery charging circuit in the device being charged that determines how much current it draws, not the charger. Of course if your device can draw 1000mA and you only use a 800mA charger then it won't charge as fast as it could but plugging it into a 1200mA charger won't make it charge any faster.