This article has missed something very important: while most cell phones charge just fine with any 500ma USB port, that does not hold true for tablets. tablets won't work with generic USB chargers.
The tl;dr explanation: use a branded charger with your tablet. iPads should use Apple branded (or logo'd) chargers with at least 1000ma output. Samsung tablets will only charge from Samsung chargers. With others, it's kind of random.
The long version:
The good news is that your mobile phone will usually work just fine when plugged in to any USB charger: 500ma will charge a typical cell phone battery in around 2-4 hours. The bad news is that tablets actually draw more than 500ma when they're running, so tablets charge very, very slowly on a generic charger, if they charge at all. The iPad's battery? It would take at least 12 hours to charge off a 500ma power source.
The problem is that there's no way of actually knowing how much current an electrical circuit can safely provide. So manufacturers have settled on the practice of putting a small voltage on the center 2 pins of the USB cable. The device senses this voltage and uses that to tell its internal charging circuitry how much current to draw from the USB jack.
The problem with that approach is that the standard isn't universally followed. Apple and Samsung both use different voltages to tell their devices that they're plugged in to a 2000ma charger.
My iPad will only reliably charge from a 2000ma power adapter that says "iPad" on the packaging. My Samsung tablet would only charge from a Samsung charger. Plug it in to a PC, an iPad charger, or my Motorola adapter, and it says "Not Charging" next to the battery.
My Nexus 7? Same deal: it charged reliably from Asus chargers (Asus makes the N7), but not from others. I finally found a $10 usb power adapter that worked in the car, but that was after trying and returning half a dozen others.
So the short version of all this: if you buy a tablet, buy a spare charger (or two) from the same company. It's the only reliable way to keep your tablet juiced up when and where you need it.