Straight needles are what most people think of when they think about knitting. They are sticks with a point on one end and a knob on the other end. These needles are used to make flat items, like blankets, scarfs, and shawls.
Circular needles, which are my favorite types of needles have two parts. There is a cord which is typically made of plastic. The other part is the needle part. This part is typically made of metal or aluminum and connected to the cord by a plastic piece that changes in diameter from the needle size to the smaller diameter of the cord. These needles can be used to make anything the straight needles can make and they can also be used to make tubes. Knitting in the round is a good way to make seamless sweaters, bags, and skirts. Another good thing about circular needles is that it seems like the weight of the knitted fabric is better distributed and there is less hand strain with these. There are also sets with interchangable needles. They come with most of the needle sizes and different lengths of cord. This is nice when you are trying to figure out what size needle to use to get the gauge of the pattern. Also, different lengths of cords enable the knitter to use the same needles to make a sleeve and the body of the sweater. The set I have was purchased at Michael's with a 1/2 off coupon. The one thing about these needles is that the needle part screws into the cord and sometimes the connection will get loose and the cord will seperate from the needle. There are other needle sets that have different ways of connecting, which I've heard isn't as likely to come undone during knititng. A company called Knit Picks has a set and there are other companies who make sets like this.
Double Pointed Needles, or DPNs as they are more commonly called are used for small things that are knit in the round. They are just sticks with points on each side. The most common uses for DPNs are socks and sleeves.
Knitting has been around for a long time, so there are different ways to measure the diameters of knitting needles. Needles are measured by at least three different systems, the US, the British and the metric. Always make sure that the needle size that a pattern calls for is the system that you think it is. The British system runs in the opposite direction of the US, so using a British Size 3 needle when the pattern called for a US size 3 would not work so well.
Knitting Needles also come in many different materials. The most common are wood/bamboo, metal/aluminum and plastic. I have found that the wood ones are good for yarns that are more slippery, like cotton. The metal ones are good for wool and things that may slide off of the needles.
What is your favorite kind of knitting needle? Have you tried the other interchangeable circular needle sets, if so, how did you like it?