What data gets passed between mirroring principal and secondary?
As noted it is based on the mode mirroring is configured to, but overall the concept of mirroring you are probably closer with statement one.

However, as noted in the documentation for mirroring:


  Database mirroring involves redoing every insert, update, and delete operation that occurs on the principal database onto the mirror database as quickly as possible. Redoing is accomplished by sending a stream of active transaction log records to the mirror server, which applies log records to the mirror database, in sequence, as quickly as possible. Unlike replication, which works at the logical level, database mirroring works at the level of the physical log record. Beginning in SQL Server 2008, the principal server compresses the stream of transaction log records before sending it to the mirror server. This log compression occurs in all mirroring sessions.


So I would necessarily say it rolls in the transactions to the database on the secondary but it applies them to the secondary. Then depending on which mode you are in would be when the transaction is committed to the database on the primary (synchronous or asynchronous).

So I would probably state it as:


  The primary server reads the transaction log of the mirrored database for a insert/update/delete occurs, that transaction is sent to the secondary and applied to the mirroring database. Based on synchronous or asynchronous the transaction on the primary (or mirrored database) is then committed.