Short answers, pulled from the story.
The Third Battle of Panipat took place on the 14th of January 1761, near the city of Panipat, approximately 97 kilometres north of Delhi. The battle lasted several days of protracted skirmishes before the decisive engagement on that date.
The Maratha Empire, led by Sadashivrao Bhau, fought against the Durrani Empire of Ahmad Shah Abdali. Abdali's coalition included the Rohilla chiefs under Najib-ud-Daula, elements of the declining Mughal Empire, and the Oudh State under Shuja-ud-Daula.
Between 60,000 and 70,000 troops were killed in the fighting, though historians dispute the exact number. The day after the battle, approximately 40,000 Maratha prisoners were massacred, and historian Shejwalkar estimated that no fewer than 100,000 Marathas in total, soldiers and non-combatants, perished during and after the battle.
The Marathas were encircled and cut off from supplies for months before the battle, leaving their troops and horses weakened by starvation. Ahmad Shah Durrani held a 15,000-strong reserve force in place until the critical moment, then deployed camel-mounted swivel cannons that devastated the Maratha cavalry. Internal disagreements among Maratha chiefs, the absence of reinforcements from Raghunathrao, and the decision to ignore guerrilla tactics recommended by Suraj Mal also contributed to the defeat.
The Marathas remained the largest empire in the Indian subcontinent and retook Delhi ten years later, in 1771, under Mahadji Shinde. Their recovery was led by Peshwa Madhavrao, but his death at age 28 triggered infighting. The Marathas were ultimately defeated and annexed by the British East India Company in 1819 following the three Anglo-Maratha Wars.
Sadashivrao Bhau was the commander-in-chief of the Maratha army at Panipat and the third-highest authority in the Maratha Confederacy after the Chhatrapati and the Peshwa. He was the cousin of the Peshwa and was appointed to lead the campaign after Raghunathrao declined the command. He fought until the end of the battle, having three horses shot out from under him, and his body was later recovered and cremated.