An Entity-Relationship (ER) diagram is a graphical representation of the relationships that exist between entities in a database. It aids in the design and comprehension of a database system's structure. The ER diagram employs specialized symbols to represent various database model components.

The following are the key symbols found in an ER diagram:

1.Entity: A real-world object, such as a person, location, or thing, about which data is recorded in a database is represented by an entity. It is commonly represented as a rectangle.

2.Attribute : An attribute is a trait or characteristic of a thing. It outlines the data that an entity can store. Attributes are often represented as ovals attached to an entity rectangle.

3. Primary Key : The primary key is an attribute (or a set of properties) that identifies each instance of an entity individually. It's highlighted within the entity rectangle.

4.Relationship: A relationship is the association of two or more entities. It is commonly represented as a diamond shape linking the linked things.

5.Cardinality: The number of occurrences of one entity that can be related to the number of instances of another entity in a relationship is defined as Cardinality. Near the endpoints of the relationship line, it is denoted by "1" or "M" (for one) and "0" or "N" (for many).

6.Weak Entity: A weak entity is one that cannot be uniquely identified by its properties alone; it must be identified through a relationship with another entity.

7. Recursive Relationship : When an entity is related to itself, this is referred to as a recursive relationship. It is represented by a relationship line connecting the entity to itself.

8.ISA (Inheritance): ISA is a type of relationship that represents generalization and specialization amongst things. It denotes the fact that one thing is a subtype of another. A triangle is used to represent the ISA relationship.

9.Attribute with numerous Values: A multivalued attribute is one that can have numerous values for a single instance of an entity. It is depicted as a double oval linked to the entity.

10.Derived Attribute : A derived attribute is an attribute whose value is derived from another attribute or entity. It's depicted with a dashed oval that's linked to the thing.

These are the fundamental symbols in Entity-Relationship diagrams. ER diagrams can be produced using these symbols and linkages to efficiently display the database's interconnections and structure. ER diagrams are important tools for database design because they ensure data integrity and provide a clear understanding of the database schema.