Short Course season runs from late August to late March. Short Course refers to competitions held in 25 yard pools.
Long Course season runs from late March to early August. Long Course refers to competitions held in 50 meter pools.
Swimmers’ Long Course times are almost always slower than their Short Course times for three main reasons:
A 50, 100, 200, etc. event in Short Course is always in yards and the corresponding event in Long Course is in meters. There are approximately 54.6 yards in 50 meters, so a 50 free (for example) in Short Course is a shorter distance to swim than a 50 free in Long Course.
In Short Course you get twice as many opportunities to flip turn and push off the wall than you do in Long Course. Once a swimmer is proficient in flip turns, this will almost always be a faster swim time for those yards than straight swimming a stroke will be.
It is usually less tiring for the swimmer to do the flip turns and streamline than it is to do straight swimming of a stroke, which in turn allows them to go faster overall during longer distance races.