When you first start out knitting you usually use single pointed knitting needles. If you made a hat with a single pointed needle when you were done with the main portion of the pattern and wanted to complete the hat, you would have to sew the two ends of the fabric together leaving a seam. As you progress, most knitters don't want the seam. This is where circular knitting needles come in. Circular knitting needles allow you to "knit in the round", which means you knit going around and around and around and there is no seam. Circular knitting needles with long length cords inbetween the needles are also good for knitting large blankets and afghans because the stitches can stay on the cord in between the needles. Single pointed needles can only hold so many stitches and if your project requires you to cast on a lot of stitches you usually start with a long length circular knitting needle.