If your database is in FULL recovery mode, the log is not truncated (truncated is a poor word, I wish they'd chosen something else) until a log backup is performed. Basically, when you put a database in FULL recovery mode, you are telling SQL Server that you want point-in-time recovery. In order to do that, SQL Server needs the full log chain from the last full (or differential) backup, and in order to do that, it does not re-use ('truncate') log space until you have backed the log up.
This is a routine operational matter for databases that are in FULL recovery mode. If you do not need point-in-time recovery, then set your database to SIMPLE recovery mode - this will automatically reuse log space, but you will only be able to recover to your last full/differential backup.
If you do need point-in-time-recovery, then you need (well, your DBA team needs) to set up a maintenance plan that performs a log backup at regular intervals.