Short answers, pulled from the story.
The Great Northern War was a conflict from 1700 to 1721 in which a coalition led by Russia successfully contested Sweden's supremacy in northern, central, and eastern Europe. It ended with Sweden's defeat and the rise of Russia as the dominant power in the Baltic region.
The anti-Swedish coalition was initially led by Peter I of Russia, Frederick IV of Denmark-Norway, and Augustus II the Strong of Saxony-Poland-Lithuania. Sweden was led by Charles XII, who also had allies including Holstein-Gottorp, Polish magnates under Stanislaw I Leszczynski, and Cossacks under the Ukrainian hetman Ivan Mazepa.
The Battle of Poltava in 1709 resulted in the destruction of the main Swedish army by a larger Russian force under Peter I. Charles XII fled to the Ottoman Empire, and the remnants of his army surrendered at Perevolochna. The defeat revived the anti-Swedish coalition and effectively decided the outcome of the war.
The war ended through a series of treaties: the Swedish-Hanoverian and Swedish-Prussian Treaties of Stockholm in 1719, the Dano-Swedish Treaty of Frederiksborg in 1720, and the Russo-Swedish Treaty of Nystad on the 30th of August 1721. Sweden ceded its Baltic provinces, Ingria, Kexholm, and a portion of Karelia to Russia, lost its exemption from the Sound Dues, and gave up most of its overseas holdings.
Charles XII was shot dead on the 30th of November 1718 while besieging the Norwegian fortress of Fredriksten. He was succeeded by his sister, Ulrika Eleonora, who began peace negotiations with the warring powers.
Ivan Mazepa was a Ukrainian Cossack hetman who had fought for Russia but defected to Charles XII of Sweden in 1708. He brought estimates of between 2,000 and 4,000 Cossacks, though Russian forces destroyed his supplies at Baturyn. Mazepa died in 1709 in Ottoman exile after the Swedish defeat at Poltava.