MySQL JOINS allow retrieving data from two or more related database tables using primary and foreign keys. There are several types of JOINs: Cross JOINs return all combinations of rows from both tables; Inner JOINs return rows where the joined columns are equal; Left and Right JOINs return all rows from the left or right table respectively and matching rows from the other table. JOINs improve performance over multiple queries by using indexing and reducing data transfers between MySQL and applications.