NOOB Q's

first of all i am a noob when it comes to Darwin and i have a couple of questions. fist off is there a way to close an application in terminal, i think there is but i don't know the button combination for it. Next how do you change directorys? and is there certain commands you can do only in a certain directory? Finaly can you check to see who has accesed your account? thanks for your help

To quit: If you know the process number, type "kill xxx" w/ xxx being the process number.

type "cd." and "cd.." to move up folder levels, and type "cd /aaa" substituting whichever directory for aaa

I'm pretty sure all commands function the same way, regardless of directory.

I don't know about the last question.