Cubism was pioneered by artists like Pablo Picasso and Georges Braque in the 20th century. It was mostly used in paintings and sculptures. In a normal piece of art you view something from one viewpoint. However, with cubism the artist attempts to depict a subject from multiple viewpoints. This results in a more abstract form. Here are three examples of cubism by Picasso:
"Les Demoiselles d'Avignon"