Up until WW2, most planes had a tailwheel or skid. Manufacturers started putting the third wheel on the front of the plane during WW2. The older planes have what is called "conventional gear" because it was most commonly used. Now, "nose-gear" planes are the most common type, but the old ones are still called "conventional gear" or "tail-draggers". The nose-gear (tricycle gear) planes are much easier to handle on the ground, especially during take-off and landing compared to conventional gear. Conventional gear planes have most of the mass behind the main wheels, so they want to turn so the mass (the tail) is in the front, causing ground-loops. Tricycle gear is more stable so they handle more easily on the ground. Conventional gear planes are considered to require more skill, although they fly just the same as tricycle gear planes. The photography equivalent is someone who does a panning image of a plane with everything sharp except for the propellor, which is ideally imaged as an entire disk.