Fins are available in a variety of blade lengths which can generally be observed as a short, medium and full length. Generally the longer the blade the better the propulsion as the fins are able to push against more water and forcing the user forward. Whilst a longer blade is great short blades are much easier to pack with the shortest fins being easily packed into a day rucksack for long hike and snorkelling trips.
Blade design refers to the technology that has gone into the blade and will determine how flexible the blade is, how much water it is capable of moving and as how much the fins will cost. A standard blade is simply a flat surface of a single material and are usually the most cost effective. Channel blades look very similar but incorporate one of more flexible panels in the blade surface to allow the blade to form a U shape that cups and focuses water into a jet for improved propulsion. Channel fins are the most common and most popular type of fin thanks to their good efficiency and relatively low cost.
Hinge and pivot blades utilise a special break in the blade that allows it to pivot to a special angle that maximises propulsion and minimises the effort required. Finally, split fins use the same principle that a birds wing use to move the snorkeller through the water by creating negative pressure on the lead side of the blades that pulls the snorkeller forward rather than push. Both hinge/pivot and split fins are comparatively expensive but offer significant advantages if you are snorkelling for long periods of time.