Research recovery model. Default - simple - contains transactions in action. Full recovery records transactions since last log backup; in case you want to restore back to before the transaction ( corruption ) occurred [need to have a full backup first!]
Two files: two states. MDF - truth. LDF intended truth.
Backing up log commits the intended truth to file (MDF)(commits the committed transactions).
Truths (transactions) completed are marked as done, so are overwritten in the log, and the file doesn't grow.
So, when you have SIMPLE recovery model and delete every row from every table in one transaction - every impacted row will be in the transaction and the file will grow. If you commit the transaction, the MDF will be empty but the size will be the same, the log will empty, the file will be same size; and you will have no data.
You cannot restore the database to the point in time before this transaction in SIMPLE recovery model ( unless you made a full backup just prior to the Great Purge) BUT if you had FULL RECOVERY mode, and a full backup, you could make a tail-end log backup; and RESTORE the full, and log backups to a point in time before the Great Purge. (STOPAT 09:59) :)
The logs there for recovery and consistency... read up on ACID transactions too.