I noticed many mentions of pty and tty in some open source projects, could someone tell me what do they mean and what is the difference between them?
tty originally meant "teletype" and "pty" means "pseudo-teletype".
In UNIX, /dev/tty* is any device that acts like a "teletype", i.e: a terminal. (Called teletype because that's what we had for terminals in those benighted days.)
A pty is a pseudotty, a device entry that acts like a terminal to the process reading and writing there, but is managed by something else. They first appeared (as I recall) for X Window and screen and the like, where you needed something that acted like a terminal but could be used from another program.