Short answers, pulled from the story.
The Cossacks originated in the Pontic-Caspian steppe of eastern Ukraine and southern Russia. By the end of the 15th century, the term was applied to Slavic peasants who had fled to the lower reaches of the Dnieper and Don Rivers, establishing self-governing communities there, though earlier roots are traced to Turkic and mixed steppe populations. The first international record of Cossack activity dates to 1492.
The word Cossack derives from the Tatar Turkic word kazak, which meant 'free man' but also 'conqueror', according to Max Vasmer's etymological dictionary. The same Turkic root produced the ethnonym Kazakh. In written sources, the name is first attested in the Codex Cumanicus from the 13th century, and in English it is first recorded in 1590.
Cossacks were the Russian soldiers most feared by the French army during Napoleon's invasion of Russia. Napoleon himself stated that 'Cossacks are the best light troops among all that exist.' Thousands of Cossacks were commended by General Pyotr Bagration for operations behind French lines, and their attacks on supply and communication routes were among the earliest examples of guerrilla warfare tactics.
The Khmelnytsky Uprising began in 1648 under Hetman Bohdan Khmelnytsky and was a rebellion against Polish and Catholic domination of the Cossacks. It was one of a series of catastrophic events for the Polish-Lithuanian Commonwealth known as The Deluge, which greatly weakened that state and contributed to its eventual disintegration roughly a century later. The uprising concluded with the 1654 Treaty of Pereyaslav, which brought most of the Cossack state under Russian rule.
The Russian Empire dissolved the Lower Dnieper Zaporozhian Host in 1775, destroyed the Sich fortress on the Dnieper, and formally transformed the remaining Cossack nations into a special military estate bound to the Tsar. Before that, the Empire had destroyed the western Don Cossack Host during the Bulavin Rebellion of 1707-1708, burned Baturyn with the deaths of 11,000 to 14,000 inhabitants after Mazepa's rebellion in 1708, and renamed the Yaik River and Yaik Host to erase the memory of the Pugachev uprising.
Between 3.5 and 5 million people across the world associate themselves with Cossack cultural identity. In the 2002 Russian Census, 140,028 people reported their ethnicity as Cossack. Cossack organizations operate in Russia, Ukraine, Belarus, Kazakhstan, Canada, and the United States, though the majority of those who identify with the culture have little direct connection to the original Cossack people.