What's inside a redo record in Oracle's Redo log?

While reading the Redo log wiki page , I was confronted with the following statement: > For example, if a user UPDATEs a salary-value in a table containing employee-related data, the DBMS generates a redo record containing change-vectors that describe changes to the data segment block for the table. And if the user then COMMITs the update, Oracle generates another redo record and assigns the change a "system change number" (SCN). In what way do change-vectors describe changes to the data? Do they contain a copy of the old and the updated data, or just the SQL statements? I also don't understand the difference between the first generated redo record and the one generated after the update is commited.