A join is a query that retrieves rows from more than one table or view. It combines rows from two or more tables based on a related column between them. The related column is usually a primary key in one table and a foreign key in another table.

Why do we need joins?
- Tables in a database are often designed to store related information. For example, a database might have separate tables for customers and orders. The customer table would store information about each customer, while the orders table would store information about each order. To analyze or retrieve information that combines data from both tables, we need to use a join operation.

Types of joins:
There are several types of joins available in SQL, including:
- Inner join: Returns only the rows that have matching values in both tables.
- Left join: Returns all the rows from the left table and the matched rows from the right table. If there is no match, NULL values are returned for the right table.
- Right join: Returns all the rows from the right table and the matched rows from the left table. If there is no match, NULL values are returned for the left table.
- Full outer join: Returns all the rows from both tables, including the unmatched rows. If there is no match, NULL values are returned for the columns from the other table.

Example:
Let's consider two tables, "Customers" and "Orders".

Customers:
| CustomerID | CustomerName |
|------------|--------------|
| 1 | John |
| 2 | Mary |
| 3 | David |

Orders:
| OrderID | CustomerID | OrderDate |
|---------|------------|------------|
| 1 | 2 | 2020-01-01 |
| 2 | 3 | 2020-02-01 |
| 3 | 1 | 2020-03-01 |

To retrieve the customer name and order date for each order, we can use an inner join between the "Customers" and "Orders" tables on the CustomerID column:

```sql
SELECT Customers.CustomerName, Orders.OrderDate
FROM Customers
INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;
```

The result of the query would be:

| CustomerName | OrderDate |
|--------------|------------|
| Mary | 2020-01-01 |
| David | 2020-02-01 |
| John | 2020-03-01 |

This shows how a join can retrieve information from multiple tables based on a common column, allowing us to combine and analyze data from different sources.