Teletypewriter originally and now also means any terminal on Linux/Unix systems. It also means any serial port on Unix/Linux systems.
Stands for pseudo terminal slave.
Main difference between TTY and PTS is the type of connection to the computer. TTY ports are direct connections to the computer such as a keyboard/mouse or a serial connection to the device. PTS connections are SSH connections or telnet connections. All of these connections can connect to a shell which will allow you to issue commands to the computer.