The exact interpretation of Waiting and Immediate can be varied to suit your practice. However the idea with the lists are:
Waiting - Patients who would like an appointment in the near future, or already have an appointment but would like a sooner appointment if one becomes available.
Standby - When you cancel an appointment you able to add it to the standby list to rebook at a later date. Useful if moving the appointment, but with Version 7, i expect most sites will just use the new Parking lot feature.
Immediate - Patients currently in the surgery waiting room in need of an appointment today (emergency etc)
To add patients to waiting or immediate simply select the list you want, then click New at the bottom of the screen. Fill out the form, selecting the patient, appointment details etc and click ok.
To then book a patient off the lists simply double click their name, you will then be taken to the planner were you simply need to right click and paste to book the appointment in the slot that is required.
Personally, i am a big fan of just using the Immediate List for everything, as you are able to visually see the notes next to all the appointments which i think makes it easier for reception when choosing which appointment to book next.(see attached picture)
Of course, depending on how large your standby list is, you might want to utilise all three lists.
Hope that helps.