An Entity$-$Relationship Diagram $($ERD$)$ is a visual representation of the data model that shows the relationships between different entities in a system. Entities are objects or concepts that have data that can be stored in a database, and relationships define how these entities interact with each other. ERDs are commonly used in database design to model the structure of a database.

Here is a basic description of the key components in an ERD:

Entities: Represented by rectangles, entities are objects or concepts in the system that have data to be stored. Examples include "Customer," "Product," or "Employee."

Attributes: Represented by ovals, attributes are the properties or characteristics of entities. For example, a "Customer" entity might have attributes such as "CustomerID," "Name," and "Email."

Relationships: Represented by lines connecting entities, relationships indicate how entities are related to each other. Common relationship types include "One$-$to$-$One," "One$-$to$-$Many," and "Many$-$to$-$Many."

Cardinality: Describes the numerical relationship between entities in a relationship. For example, in a "One$-$to$-$Many" relationship between "Author" and "Book," one author can have many books, but each book is associated with only one author.

Primary Key: Denoted by underlining an attribute, the primary key is a unique identifier for an entity. It uniquely identifies each record in the entity.

Foreign Key: Represented by an attribute in one entity that refers to the primary key in another entity. It establishes a link between the two entities.

Here's a simple textual representation of an ERD:

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Customer $($CustomerID$,$ Name, Email$)$

$|$

$|-----<$ Orders $($OrderID$,$ OrderDate, TotalAmount$)$

$|$

$\text{'}-----<$ Payments $($PaymentID$,$ PaymentDate, Amount$)$

Product $($ProductID$,$ ProductName, UnitPrice$)$

$|$

$\text{'}-----<$ OrderDetails $($OrderDetailID$,$ Quantity$)$

In this example, there are two entities: "Customer" and "Product." The "Orders" entity has a relationship with "Customer," and the "OrderDetails" entity has relationships with both "Orders" and "Product." This illustrates how entities are related in a basic sales system.

Note: The actual structure of an ERD can vary based on the specific requirements of a system or database. Complex systems may involve more entities, relationships, and attributes.