Programmed instruction was introduced in the 1950s as a way for students to learn on their own without a teacher through specially prepared books or equipment. It breaks content down into small sequential steps with frequent responses from the student and immediate feedback. There are two main types - linear programming which presents information in a fixed orderly sequence, and branching programming which allows for different paths. Programmed instruction aims to help students learn at their own pace, present content logically, and allow them to study independently.