Short answers, pulled from the story.
The Battle of Eylau, fought in February 1807 near the East Prussian town of Preussisch Eylau, was a bloody and strategically inconclusive clash between Napoleon's Grande Armee and the Imperial Russian Army under General Levin August von Bennigsen. It was the first serious check to the Grande Armee and badly shook the myth of Napoleon's invincibility. Both sides suffered enormous casualties without either achieving a decisive result.
Napoleon commanded the French Grande Armee. The Russian forces were led by General Levin August von Bennigsen. Key subordinate commanders included Marshals Murat, Davout, Soult, Augereau, and Ney on the French side, and Generals Bagration and Barclay de Tolly on the Russian side. A Prussian corps under Von L'Estocq, whose chief of staff was Gerhard von Scharnhorst, arrived late in the battle and played a decisive role.
Casualty figures vary widely among historians. Russian losses were estimated at between 15,000 and 20,000 killed or wounded, plus around 3,000 captured. French losses ranged from 10,000-15,000 in lower estimates to 25,000-30,000 in higher ones; German historian Horst Schulz calculated 29,643 French casualties total, including 4,893 killed. Bennigsen himself estimated his own dead at up to 9,000.
With the French centre on the verge of collapse, Napoleon ordered Marshal Murat to launch all 11,000 riders of his cavalry reserve at the Russian army. The charge swept through Russian infantry, broke through two lines of infantry, and rode through the Russian gun line before exhausting itself some 2,500 yards into the Russian position. Murat lost between 1,000 and 1,500 troopers but bought enough time for Davout's corps to deploy in strength on the Russian left.
A blizzard descended just as Augereau's VII Corps advanced to attack the Russian centre on the second day of fighting. The corps lost all sense of direction, veered off course, and struck the Russian line at the wrong point, coming under fire from their own artillery and then point-blank fire from a massive 70-gun Russian battery. Augereau's official casualty count from the attack was 5,200 men. Heavy snow also delayed Ney's arrival, as it muffled cannon fire and left him unaware of the battle until a messenger reached him.
Bennigsen decided at 23:00 to withdraw, leaving Napoleon in possession of a battlefield covered with dead and wounded. Napoleon offered Prussia a separate peace through General Bertrand, but Prussia rejected it to maintain its alliance with Russia. Hostilities continued until the French victory at the Battle of Friedland in June 1807 forced Tsar Alexander I to negotiate, and both sides signed the Treaties of Tilsit, which stripped Prussia of almost half its territory.