A schema, or scheme, is an abstract concept proposed by J. Piaget to refer to our, well, abstract concepts. Schemas (or schemata) are units of understanding that can be hierarchically categorized as well as webbed into complex relationships with one another.
For example, think of a house. You probably get an immediate mental image of something out of a kid's storybook: four windows, front door, suburban setting, chimney. However, if I were to amend the object's name slightly, your scheme would shift to a more refined version. How about: Shotgun house? One door, maybe no front windows, low income setting. Mansion? Multiple windows, side entrance for the help, sweeping front drive.
That is a simple example, but our schemas get incredibly complex as we learn more about the world, and particularly as we become experts in a field. The more we know, the bigger and more complex our schemas become. However, the more we know, the easier it is to remember new information related to the schema - because there is more pre-existing information in our heads that we can relate - and thus attach - it to.
For students, their schemas pretty much amount to what they already know about a concept. They may have learned it in other classes or through their own experiences. What they "know" may be incorrect. Our job is to either expand or correct their schemas about important concepts in our fields.
However, no information will attach to their schemas if they we aren't thinking about that schema when the information comes in. Let's say you know a fair bit about prehistoric fossils and take a trip to the Gray Fossil Site. While the guide explains a fossil find that is new to the field (and thus to you), yet you are thinking about the design and outlay of the museum, the information will go in one ear and out the other.
Therefore, the most important rule for teaching based upon Schema Theory is
Make sure students' existing schemas are up and running at a conscious level
1. Use advance organizers.