Nintendo has designed the back of the Wii U GamePad (that's the one with the power-sapping screen) with regular screws, allowing Joe Public the ability to access the rechargable battery and change it. Sounds like a small concession, but it's not the norm for such devices, as mobile electronics such as phones, iPads and DualShock 3s all feature sealed batteries that you aren't meant to be able to change yourself.
It's more necessary for Wii U than with these devices, as the Wii U GamePad's estimated battery life from a single charge is between 3-5 hours. Forget to put it back on charge even once and you're in danger of it running out during your next play session, forcing you to play while connected to the base unit with a cable. The solution? Have a spare battery on standby.
The replacement batteries will be available directly from Nintendo itself but prices haven't been announced yet. We should point out that nobody is being forced to spend extra money on spare batteries, although Nintendo does estimate battery life in the unit will be reduced to 70% performance after 500 charges. That's still thousands of hours in, but close enough that it may happen during the console's lifetime.