When you use circular needles, you're pretty much knitting the same as on straights. In fact, many of us use them as straights rather than ever using regular needles. If you use them as straights--everything is the same.
When you use them to knit in the round, you're making a tube, with all the right-side stitches on the outside and wrong side on the inside. The length of the cable determines how many stitches you can fit. You can't, for example, make a hat on 32-inch needles unless you have a 32-inch head.
What is it you're planning on making?