Lord Balarama is the first personal expansion of Lord Krsna, the Supreme Personality of Godhead. All other incarnations expand from Him. In Lord Krsna's pastimes, He plays as Krsna's older brother. Together Krsna and Balarama enact many pastimes as cowherd boys in the land of Vrndavana. Lord Balarama carries a plow and club and is known for His great strength.
(See Sri Caitanya-caritamrta, Adi-lila, Chapter 5.)