"Schema" is usually used to mean any cohesive thought or behavioral pattern. Any collected or organized group of ideas, habits, or knowledge can be classified as a schema (Wikipedia). New knowledge is stored in existing mental structures that file and categorise it based on what is already known; this knowledge is then brought out in the mode of the structure to interpret new situations. A negative example of schema is prejudice based on pre-conceived notions; without the mechanism to think past the existing mental structure, all experiences are filtered through the negative schema, often reinforcing it. To alter the schema, one must consciously learn new patterns of thought or new knowledge that contradicts the old.
Schema theory is generally accepted to have been created by psychologist Frederic Bartlett in the early 20th century; his view focused on the subjective mental experiences that make up each person's (relatively) objective reality. His work was expanded on by R.C. Anderson and Jean Piaget.