Tsar was the title used for the rulers of Russia from 1546 to 1917. In 1721 Peter I adopted the title Emperor (Imperator), by which he and his heirs were recognised, and which came to be used interchangably with Tsar.
The word tsar (sometimes transliterated less correctly according to modern standards as "czar" or "tzar") is from the Latin "Caesar" by way of the 11th century Russian tsisari (later tsari), from Old Slavonic tsesari.
Tsarina is used for an empress, tsarevna for the daughter of a Tsar or Tsarina, and tsarevich for a son. The Patriarchs, heads of the Russian Orthodox Church, acted as leaders of Russia at times, as during the Polish occupation and interregnum of 1610-1613.
The now deprecated spelling czar originated with the German diplomat Baron Sigismund von Herberstein, whose Rerum Moscoviticarum Commentarii (1549) (translated into English as Notes on Russia) was the main source of knowledge of Russia in early modern western Europe.
The spelling tsar has been accepted in English for the last century as the correct usage. French adopted this form during the 19th century, and it became frequent in English towards the end of that century, following its adoption by the London Times newspaper. (see the Oxford English Dictionary, 2nd edition).
Some of the Tsars were:
Tsar was also the title of the rulers of Bulgaria in 893-1014, 1085-1396 and 1908-1946, and of Serbia in 1346-1371.
See also the history of Russia, history of Finland, history of Belarus, history of Ukraine, and lists of incumbents.