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Recovery models are designed to control transaction log maintenance

simple - No log backups

Automatically reclaims log space to keep space requirements small

full - Requires log backups

Can recover to a specific point in time, assuming that your backups are complete up to that point in time

The "Full" recovery model tells SQL Server to keep all transaction data in the transaction log

    until either a transaction log backup occurs or the transaction log is truncated.

bulk-logged - Requires log backups

Reduces log space usage by using minimal logging for most bulk operations (Point-in-time recovery is not supported.)

Simple Recovery Model

After Switching from the Simple Recovery Model

[1] Immediately take a full or differential database backup to start the log chain.

 * takes effect only after the first data backup

[2]  Schedule regular log backups

 * Log backups allow the transaction log to be truncated.

   If you do not back up the log frequently enough, the transaction log can expand until it runs out of disk space.

After Switching to the Simple Recovery Model

 * you break the backup log chain
    Therefore, we strongly recommend that you back up the log immediately before switching

 * truncate the inactive portion of the transaction log