In short circumpolar stars are stars that never set over the horizon line. over time all other stars appear to rotate around these stars.
When considering a star trail or a star time-lapse one of the main goals (IMHO) is how to find the point where all the stars are rotating on a virtual axis. For the north and south hemispheres these are obviously different stars. Here are a couple of tips.
Star Walk
One iPhone App to help you get your bearings when planning for a star time-lapse is called Star Walk. This can show you everything you need to know about which stars are where and you can even fast forward time to show you the star movement over the next few hours. This will help you locate a relative centre point and also show you whether the moon will rise and cross in your shot, which could ruin your exposure and an entire night of shooting.
General Tip
If you don’t have the use of an iPhone or a data connection in your location like I didn’t have at Lake Mungo then your best chance is to shoot directly south if you’re in the southern hemisphere or directly north in the northern hemisphere. This way you are more than likely to catch the circumpolar stars in your shot and have a great star trail or time-lapse.