The Raspberry Pi Zero USB port may be used as an USB serial port. Here is how I did it.

Modify /boot/config.txt

One will first add a device tree overlay (see Using SPI displays with a Raspberry PI). By adding dtoverlay=dwc2 in /boot/config.txt, one loads a device tree overlay which modifies the USB OTG configuration to make it more suitable to our needs.

Modify /boot/cmdline.txt

 We will then modify the kernel command line and add modules-load=dwc2,g_serial at its end. This loads the serial gadget.

Starting a getty on the newly created serial port

Simply type sudo systemctl enable This email address is being protected from spambots. You need JavaScript enabled to view it. . You will then be able to connect a standard USB cable and the host will see a serial device through which you may get a login prompt.

Wait, there's more!

The Raspberry Pi Zero (and of course the Zero W) may appear as many other USB devices. See this Gist: https://gist.github.com/gbaman/50b6cca61dd1c3f88f41 and have fun!

Come back, there's even more!

http://isticktoit.net/?p=1383 has a very complete article on the subject of USB Gadgets. The kernel documentation is of course also a good read!