Psuedo-TTY's come in pairs, one end for the application to connect to, the other for whatever is consuming the output or producing input to the application. So, yes, of course they are not pipes but they are similar in that you have two end-points. So pipes and pseudo-tty's have quite a bit in common, thinking of a pseudo-tty pair as a pipe that knows how to deal with a bunch of terminal specific ioctl calls is a pretty good approximation.