In SQL, joins are operations used to combine rows from two or more tables based on related columns. Joins allow you to retrieve data from multiple tables in a single query by establishing relationships between the tables using common columns.

There are several types of joins in SQL:

1. Inner Join: The most commonly used join, an inner join returns only the rows where there is a match between the columns in both tables being joined. It combines the rows based on the matching values in the specified columns.
2. Left Join (or Left Outer Join): A left join returns all the rows from the left (or first) table and the matching rows from the right (or second) table. If there is no match, NULL values are included for the columns from the right table.
3. Right Join (or Right Outer Join): A right join is the opposite of a left join. It returns all the rows from the right (or second) table and the matching rows from the left (or first) table. If there is no match, NULL values are included for the columns from the left table.
4. Full Join (or Full Outer Join): A full join returns all the rows from both tables, including the unmatched rows. It combines the rows from both tables and includes NULL values for the non-matching columns.
5. Cross Join: A cross join, also known as a Cartesian join, returns the Cartesian product of the two tables. It combines each row from the first table with every row from the second table, resulting in a combination of all possible pairs of rows.