Kaiser is a German title that means “emperor” and derives from the Roman word for “Caesar” from back in the times of the Roman Empire. Its Russian equivalent is "Tsar".
Charlemagne became the first German Emperor of the Holy Roman Empire of the German Nation in 800 AD when he was crowned by the Pope. This first "Reich" lasted for over a thousand years (800 - 1806) but near its end was nothing more than a loose federation of states. The last Emperor of this first German Reich, Franz II, Archduke of Austria, abdicated from this title under pressure from Napoleon in 1806, but had crowned himself Emperor of Austria in 1804.
The second German Empire, founded in 1871, had three Kaisers. Due to political reasons (concerns of the other German states as well as of states like Austria, who had belonged to the first German Reich, but weren't part of the second), they were styled as Deutscher Kaiser (German emperor) instead of Kaiser von Deutschland (emperor of Germany). The last one was Kaiser Wilhelm II, who abdicated in 1918, and went into exile in the Netherlands. The last Austrian Emperor was Kaiser Karl I, who assumed the throne in 1916 from Kaiser Franz Joseph I, abdicated in 1918, and went into exile in Switzerland. He was beatified by the Roman Catholic Church in 2004.
The third German Empire, called the Third Reich, would take place under Hitler and would be the last, but there were no titles of Kaiser. That term had been put to rest.