A Schema shows the organisational structure of a database. It should show the entities (the tables in the database), and their characteristics (ie their properties/attributes/fields). It should clearly identify the primary key in each table and the links and relationships between tables. 

A Primary Key is a grouping or arrangement of characters (letter, numbers etc) which is used to identify a unique record in a database.

A Foreign Key is a field in a relational table that matches the primary key column of another table. This foreign key can be used to cross-reference tables.