Brew In A Bag (BIAB) brewing, is an inexpensive way for homebrewers to transition to all grain or partial mash brewing. BIAB also allows shorter setup, brewing and cleanup times.
The BIAB method involves using a grain bag set in the brew pot to mash the grains, followed by a sparge step where the bag is removed from the pot and the remaining wort is boiled as you would any other beer.
Brew in a bag is usually done using a single step infusion mash, the same profile most all grain brewers use. This involves preheating the water in the mash tun to a predetermined temperature before adding the grains. What differs from traditional methods, in BIAB, the entire pre-boil volume of water is used for the mash.