Final answer:
An online library management system requires an ERD, relational schema and HTML/PHP source code for database management. The ERD illustrates the database structure, whereas the relational schema outlines detailed relationships between the tables. Implementing code in HTML and PHP provides user interfaces and database interactions.
Step-by-step explanation:
Designing an online library management system involves several steps including setting up the Entity-Relationship Diagram (ERD), creating a relational schema, and developing the source code using HTML and PHP to create and manage the database.
An ERD is a visual representation of the database which helps to understand the relationships between different entities such as books, users, and loans.
The relational schema is a detailed description of database tables and the links between them. Developing the HTML and PHP code allows for the creation of an interface for users and administrators to interact with the library system.
It's important to note that, being a complex project, each step must be executed with dedication and comprehension. The scope of this answer is too limited to provide full-fledged source code, but we can discuss general components and concepts.