The two shapes
A straight needle is a rod with a point at one end and a stop at the other, sold in pairs. The whole piece of knitting hangs off the two rods, and the rods are held out from the body. A circular is two short tips joined by a flexible cable. It knits in the round, which straights cannot, and it knits flat just as well: work across to the end of the row, turn, and knit back, exactly as on straights, with the cable doing the holding.
Everything else follows from that.
Why most knitters end up on circulars
The weight sits in your lap. On straights, a sweater back or a blanket hangs off the needles and your wrists carry it. On a circular the fabric rests on the cable and the cable rests on you; a blanket that is impossible on straights is comfortable on a 40" circular. This is the reason knitters with sore wrists or shoulders switch, and the reason they do not go back.
No width limit. A 14" straight holds about 14" of knitting bunched up; a 60" circular holds a blanket. The circular length guide covers which length for which piece.
One needle for flat and round. A hat, a cowl, sleeves, the yoke of a sweater — anything round — needs a circular or double-points. If you own the circular already, a flat project does not need a second set of needles.
Travel and small spaces. Circular tips are short, nothing sticks out to the sides, and nothing slides off the far end on a bus. Airlines that eye a 14" metal straight rarely blink at a bamboo circular.
Where straights still win
Learning. The first rows are easier to see on straights: two rods, the stitches move from left to right, nothing twists. Many teachers start beginners on a short pair of bamboo straights for this reason, and move them to circulars once the hands know what they are doing.
Small flat pieces. A scarf, a dishcloth, a baby bootee knit flat, a swatch — on a 10" or 12" straight these are quicker to pick up and put down than a circular whose cable wants to curl.
Grip and rhythm. Some knitters hold a straight needle tucked under the arm or against the hip and work with one hand only, a fast old technique that a circular does not allow. And some simply like the feel of a long needle; it is a preference, not a mistake.
Knitting flat on a circular
Cast on as usual, knit to the end of the row, turn the work, and knit back — do not join. The only difference from straights is that both ends of the row are the same needle. Choose a cable long enough to hold the piece comfortably without stretching it (24" to 32" for a sweater piece, longer for a blanket) and a little longer than you think; a flat piece can always be shorter than the cable but never longer than the stitches will stretch. A circular with a stiff cable will fight you when turning; the cables on addi and Crystal Palace circulars are supple and lie flat.
Metal or bamboo
This is a separate choice from circular or straight, and it matters more — the needle material guide goes into it. Metal tips — addi Turbo and Rockets — are slick: stitches fly, which is what you want for plain stockinette in a smooth yarn and what you do not want for a slippery silk or a loose knitter whose stitches fall off the needle. Bamboo and rosewood grip the yarn a little: slower, warmer in the hand, quieter, and kinder to beginners and to lace. Many knitters own both and pick by yarn.
What we stock
Circulars: addi Turbo (nickel-plated, the fast ones) in 16", 24", 32", 40" and 60"; addi Rockets and Rocket2 Squared (sharper lace tips) in 24" to 40"; Crystal Palace bamboo in 16", 26" and 35"; Reynolds rosewood 24". addi Clicks is the interchangeable set — one pair of tips, cables of every length. Straights: Crystal Palace bamboo in 10" and 12", and Reynolds rosewood 10". The needle sizes chart is for matching any of them to a pattern.
Which to buy first
One circular in the pattern's size, 24" or 32" long, in bamboo if you are new or knit loosely and in metal if you want speed. It knits the hat, the scarf and the sweater, and it is the needle most patterns are written around. Add straights later if you find you like them — a 12" bamboo pair costs little — rather than the other way round.