SQL INNER JOIN is used to retrieve records that have matching values in two or more tables. It combines rows from these tables based on a related column, typically a primary key in one table and a foreign key in the other. The INNER JOIN only returns rows when there is at least one match in both tables, making it ideal for extracting meaningful, relational data.

For example, if you have a Customers table and an Orders table, an INNER JOIN can show all customers who have placed an order, excluding those who haven't. The basic syntax is:

SELECT *
FROM Customers
INNER JOIN Orders
ON Customers.CustomerID = Orders.CustomerID;

INNER JOINs are fundamental in relational database management systems (RDBMS) such as MySQL, SQL Server, and PostgreSQL, allowing developers and analysts to build powerful queries for reporting, analytics, and data integrity. It’s one of the most commonly used SQL joins.