How do I use UART serial port on the Pi-2 bus?

[SvdSinner]

>$ screen /dev/ttyS2 115200

If you're using a USB UART adapter make sure it's set up to run on 3.3 volts.

I will add :
  uart2:  /dev/ttyS2
  uart3:  /dev/ttyS3
  uart4:  /dev/ttyS4

You will probably want to do one of the following:

   sudo chmod 0777 /dev/ttyS2

or

   sudo usermod -aG dialout  username

Fantastic!  Are there any more general guides on how to interface with the various IO ports that don't really exist in the PC linux world?

I've dealt with microcontrollers, and I've dealt with PCs, but this is my first venture into Linux where I will deal with GPIO, UART, I2C, etc.