In MySQL, the binary log (binlog) is a record of all changes to the database, including data modifications and structural changes. The binlog is used for several purposes, including replication, point-in-time recovery, and crash recovery.

The format of the binlog is determined by the value of the binlog_format system variable. MySQL supports several different binlog formats, including:

1. Row-based: In this format, the binlog records the actual rows that were changed by each statement, along with the necessary metadata (e.g. the table name and columns). This format is useful for replication and point-in-time recovery, since it allows the changes to be reapplied to another server or to a specific point in time.

2. Statement-based: In this format, the binlog records the actual SQL statements that were executed, rather than the individual rows that were changed. This format is more compact and efficient than the row-based format, but it does not provide as much information about the changes that were made, and it may not be suitable for certain applications (e.g. replication or point-in-time recovery).

3. Mixed: In this format, the binlog uses a combination of row-based and statement-based formats. Statements that affect a small number of rows are recorded in the row-based format, while statements that affect a large number of rows are recorded in the statement-based format. This allows for a balance between efficiency and accuracy, but it may not be suitable for all applications.

The binlog_format system variable can be set at runtime, so you can choose the appropriate format for your specific needs and requirements. It is important to choose the right format since the format of the binlog can impact the performance, reliability, and flexibility of your MySQL server.