I'm making a sweater, and I'm not sure when to use circular needles versus regular ones.?

I'm teaching myself how to knit, and I want to make a sweater. The pattern doesn't specify...and I'm not sure when to use circular needles versus regular ones. Do I really need the circular ones? I'm using Charmed Knits -- you can see the preview of the page i'm using http://www.amazon.com/gp/product/0470067314/ref=cm_cr_error