The LDF file contains the tranaction logs. It is used if your recover the database from a backup.
The transactions should get comitted when you backup your SQL database (usually via a maintenance plan). The "recovery model" of the database also affects if you have a transaction log (a simple recovery model should create a very small LDF file).
Try right clicking the database in Enterprise manager and performing a backup, this may reduce the log file. If that doesn't work, post back and we can look at some other options.