A terminal is the end of a line. So, back in the day when the computer was a mainframe serving many user accounts, what you'd be sitting at with your keyboard and display would be a terminal.
A terminal emulator is when you're using a computer (a Turing machine) to provide the function of a terminal in software. This usage would typically come up because the computer would be 'imitating' a particular type of terminal in order to communicate with the mainframe.
A very popular terminal is/was the VT100. So if I telnet right now to the server of the local Freenet (if they still exist) I'd be using VT100 emulation.
A fonte e mais explicações estão no Terminal ou emulador de terminal? .