schemas are organised packets of information that are built up through experience and stored in the long-term memory.
we have schemas for a vast number of people, places and activities - these are generally derived from past experiences, but can be refined through further interactions with people and the worldaround.
schemas are important because they can help us to make sense of new experiences.
for example, the halo effect suggests that if our schema for an individual contains positive impressions, e.g. the person is friendly, we are more likely to believe they have other positive characteristics as well, e.g. they are also intelligent.