Have a read through this article - towards the end about BULK LOGGED.

To summarise (from the article by Paul Randal):

https://technet.microsoft.com/en-us/magazine/2009.02.logging.aspx#id0060077

The BULK_LOGGED recovery model has the same transaction log truncation semantics as the FULL recovery model but allows some operations to be partially logged, which is called being minimally logged.....only the allocation changes are logged, which drastically reduces the number of log records thus reducing the potential for transaction log growth.