Most knitters start out knitting with straight knitting needles, also known as single points. These firm, straight needles have a point on one end and some kind of knob on the other to keep the stitches from slipping off that end of the needle.
These are usually the first choice of new knitters because they're available everywhere, they're inexpensive (at least in the most basic forms) and they look a little less intimdating than circular needles.
Straight needles -- and any other needles, for that matter -- can be made out of many different materials, including aluminum and other metals, wood, bamboo, plastic, glass and bone. Plastic needles are inexpensive and great for kids. Aluminum needles are also cheap, but they can be cold and make that irritating clanking sound when you work with them.
My preference -- which is completely personal, just as your choices will be -- is to use wooden or bamboo needles. They warm up in your hands, have a little flexibility and are quieter, and they just feel like a more natural product.
Straight needles can be purchased in a couple of different lengths, usually 9 or 10 inches and 12 or 14 inches, depending on the manufacturer.
Straight needles are great for small projects like sweaters worked in pieces, dishcloths, afghan blocks and hats and mitts worked flat. Some knitters don't like the inflexibility of straight needles or the awkwardness of holding a large knitting project on one needle. They can be uncomfortable to work with because you are holding the weight of the project more than the needle is.