The log records the changes since the last backup. You can restore your database from just the backup, but the logs allow you to restore modifications made since your last backup.
Please read Books Online for all the details.
I've read the BOL, let me explain it
i have a 2 GB database, on FULL model, the transaction log is 800 MB with no automatic grow file.
I scheduled a complete backup and 2 logs backups during the day, my Transaction log filled because my night log backup failed due the media, i thought that complete backup also backs up log but i found it that doesn't.
This is how is supposed to work right? if my backup logs fail no matter how many Complete backups i made, my transaction log will fill.