In addition to the typical Euro and Oval profile cylinders, we have Rim cylinders ('yale lock' barrels) seen on front-doors all throughout the UK, and screw-in cylinders often found on shopfront aluminium doors.
In addition to these popular replacement cylinders, we also have Scandinavian Oval cylinders (often referred to simply as 'Swedish Oval cylinders'), mini-oval cylinders and a whole range of the specialist Kaba 20 cylinders and the replacement Adams Rite cylinders used on some of ASSA Abloy's patio door locks.
The most popular range here is the "Rim Cylinder" range. If you have a "Yale Lock" on your front door, where the lock body sits on the inside of the door with the little thumb-turn knob and the holdback snib, then these are the types of cylinders these locks typically use. From the outside you just see the circular face of the rim cylinder and potentially a finger-pull surround fitted around it. If you're looking to replace your cylinder on this type of lock, then look at the rim cylinders range.
The second most likely choice is for screw-in cylinders. These in principle look similar and function very similarly to rim cylinders, but the difference is the body is threaded and these screw into the lock housing morticed into the door. These are typically most common on metal doors on commercial buildings or communal front-doors on blocks of flats and so on.
For everything else, check the "Miscellaneous Cylinders" range. This as mentioned includes scan-oval and mini-oval cylinders, as well as the Kaba and Assa ranges of cylinders.