TTYs refer to the input devices including the virtual consoles and the serial ports (RS-232 and USB).
If refered to as tty1-6 they generally refer to a command line interface accessable to the system. If it refers to tty7, it is generally the terminal that the X-server (graphical interface) is running on. If it refers to /dev/ttys01, then it is refering to a serial device at 01, generally a standard serial port. Modems are addressed at the serial port they occupy. If it refers to someting like /dev/ttyUSB01, then it is refering to a USB device.
Remember, everything on a Linux or Unix system is addressed as a file in the system architecture.