1. SQL Maintenance plan can be configured to delete files older then X days, check up under "Maintenance" tab for existing maintenance plan.
2. The*.bak files are probably the full backups and the *.TRN are the transaction log backups, probably you can delete the oldest BAK file and the TRN's files that follows until the next BAK files.
Take a look in the following tutorial and I think after that the backup picture will be clearer for you - http://www.dbsnaps.com/articles/sql_backup_restore
Oded