CSE runs a booking system in all of the non-restricted laboratories and in the thesis lab. All students are supplied with tokens to make bookings; the number and type of tokens you receive is dependent on the courses you are enrolled in. The minimum period that can be booked is 30 minutes: if you only need a computer for 5 minutes you're probably better off using one of the bookterms. The longest period you can pre-book is 3 hours (since no student receives more than 6 tokens) but, since each token is returned to you at the end of the 30 minutes, in practice you can keep on extending the booking until the lab closes or another booking intervenes.
The basic command is book but as its syntax is somewhat obscure most students find the GUI frontend to book, tkbook, much easier to use. When you run tkbook, it will look something like this, listing the labs and timeslots available to you. A timeslot marked "." has workstations available for booking; a timeslot marked "Full" may be fully booked, or the lab may be closed at that time (which is rather confusing).
There is no direct equivalent of book in the Windows labs. Students in the Windows labs should use the web interface to book as below, or ssh to login.cse.unsw.edu.au and run book/tkbook.
To unbook a timeslot in tkbook, click on it and then click on "Unbook". To unbook a slot from the web interface, just click on it. You should unbook any booking you can't attend as you will be marked as a defaulter if you just forfeit it.