Circular Needles have 2 needle tips connected by a cord. Also called circulars or circs, they are popular for knitting projects that are flat or in-the-round, as well as magic-loop style (which is a money saver as you won't need to buy as many different cable lengths). You can get circular needles as fixed, interchangeable (IC), or convertibles.
Fixed circular needles come in 1 piece and have no parts to connect, so the cable is fixed to the tips. They come in a variety of lengths from as short as 9 inches (25cm) in lenght to as long as 60 inches (150cm) for fixed needles.
Interchangeable Needles come with tips sold separately from the cable, so that you can choose what length of cable you would like. Most brands that make tips and cables that connect are considerate enough that they ensure that all sizes of tips are compatible with all sizes of cables, which really is convenient. You know they are interchangeable when the cables list lengths but not sizes such as US (or mm) of needle they work with.
Convertibles: Some needle and tip combinations have cables that are not compatible with various sizes of needles, so therefore they are convertibles, not interchangeables. These ones the cable says what length it is and also what size needle it works with. Yes, this does get complicated, sorry.