The log contains a list of changes since the last backup, it is used by SQL to recover changes that may otherwise be lost due to system failures etc.
The log can also give you flexability with your backup / restore strategies as it can be backed up frequently without imposing too much in th way of overheads on your system.
If you really, really, really don't need it then you can set the DB to simple logging and it will not be used. However bear in mind you almost definately will regret this on a production server.
The best way to mange it's size is to mke sure you are doing regular online SQL backups as the default is to clear the log on a succesful backup.