A schema defines the logical structure of the data, and it is independent of the physical storage of the data.
There are different types of schemas in a DBMS, such as:
Physical schema: describes the physical organization of the data on the storage media, including the file organization, access methods, and storage structures.
Logical schema: describes the logical organization of the data, including the tables, columns, relationships, and constraints that make up the database.
External schema: describes how the users view the data, and it is usually a subset of the logical schema.
A schema is usually represented using a data modeling language, such as ER or UML, which is used to create a visual representation of the database structure. Schemas are used by the DBMS to optimize the performance of the database and to ensure data integrity and consistency.