Burrows are orc structures that served dual purposes: they act not only as farms for the harvesting of various roots and crops, but also as fortified bunkers for the peon workers to hide in during times of attack. Increasing numbers of these structures provide food to support the amassing of additional troops. Once Thrall began ordering Burrows to be built instead of Pig Farms, he saw how they could be beneficial to his workers. Spears, bows, and other long-ranged weaponry are stored within a burrow with it's store of food. Peons are not... particularly effective... with these weapons, but peons are better at fighting behind a stone wall, rather than on the front lines.
Burrow armor has been increased from 2 to 5 and damage has been reduced from 1d8+33 to 1d5+22.
Patch 1.10 (2003-07-03): An "Occupied" graphic now appears over Orc Burrows, Goblin Zeppelins, Entangled Gold Mines, etc. This allows players to quickly tell how many units are currently occupying a structure or transport.