Swimming trunks are lightweight shorts that stop at varying distances above the knee. Most are made of nylon and include a mesh lining for quick-drying qualities. They can be identified by their loose, relaxed fit and are typically not used for competitive swimming as they can create drag in the water.
Swim trunks are a form of swimsuit - clothing worn specifically for swimming. As such, they are usually made of materials specifically designed for being wet and remaining comfortable and hydrodynamic to not impede the swimmer.