Loads are caused by lots of different conditions as described later. Tension is a load which stretches or lengthens a structure. Easily demonstrated by pulling a piece of clay. Compression is a load which shortens or compresses a structure. Shear makes the structure slide one piece over another.
Torsion twists a structures.
Loads come from the forces which are placed on a structure. Each piece of a structure has weight. These loads are referred to as Dead Loads. Live loads are loads put on the structure after it is built. Live loads can come from people, cars, weather - like rain, snow, wind, waves, earthquakes.