If you are talking about domestic travel then in a nutshell the cheaper the ticket the more restrictive it is (is non-refundable, fees for changes etc). The more expensive the ticket the more flexible it is (changes may be permitted without fees and some are even refundable). You really just have to read through the fare rules at time of booking and ask yourself how likely it may be that you need to change or cancel your plans. Personally Virgin Blue are a more people friendly airline, Jetstar treat their passengers as a number and they have a very 'we don't care' attitude (well that's been my experience anyway).

Good luck and have a good time where ever you decide to go.