Hulless barley is free threshing (the grains are easily detached from the glumes), while most barley varieties are hulled. Hulless barley was grown a lot in ancient times, but later hulled barley took over, though hulless barley was still grown here and there. For small scale growing hulless barley is practical, since it is easier to extract the grains.
As the name indicates, two-row barley has two rows of grains and awns on the spike, since two of the three flowers in each spikelet are sterile and awnless.