I've been reading a guide on hardening linux and quite often its referred to tty's. I googled but the response i got wasnt great and im wondering if someone could give me an explanation.

Is it a hardware or software thing?

Thanks,
NNP

man tty

ttys are the general terminal interface.
If you're at the command prompt, Ctrl+Alt F1-F6 are different ttys.
tty01, 02, etc..
When sshing in, your terminal is assigned a tty address.

That's my RTFM answer, hope it helps.
-Jason

Hmm I was under the impression it had something to do with modems. Apparently not.

http://www.google.com/search?q=define:TTY

Plenty of resources there, I believe they're related to modems too.
-Jason

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.