As is known, database schema is a collection of database rules like data type and data relationship. However, this is still a little abstract. So what is database schema in one sentence everyone could understand it clearly.
The wikipedia page on Database schema is quite good for understanding the concept.
A database schema of a database system is its structure described in a formal language supported by the database management system (DBMS) and refers to the organization of data as a blueprint of how a database is constructed (divided into database tables in the case of Relational Databases). The formal definition of database schema is a set of formulas (sentences) called integrity constraints imposed on a database.