Tsar (Russian царь, from Latin Caesar, cognate with German Kaiser; also spelt Czar in English borrowed from Hungarian) was the title used for the rulers of Russia from 1546 to 1917 (the Imperial Russia). It was adopted by Ivan IV as symbolic of a change in the nature of the Russian monarchy. In 1721 Peter I adopted the title Emperor (Imperator), by which he and his heirs were recognised, and which came to be used interchangeably with Tsar.
Often the word tsar could be translated with emperor, and reversely, in Slavic languages, emperor is oftenly translated with tsar (e.g. The Japanese emperor (mikado) becomes literally "Japanese tsar").
Tsaritsa is the term used for an empress, though in English contexts this seems invariably to be altered to tsarina. Tsarevna is the term for a daughter of a Tsar or Tsarina, and tsarevich for a male heir apparent. (Sons who are not heirs are called Grand Dukes.)
Territory over which a tsar rules is tsardom.
A note on spelling and pronunciation
The spelling tsar is the closest possible transliteration of the Russian using standard English spelling. Both czar and tsar have been accepted in English for the last century as a correct usage. French adopted the form tsar during the 19th century, and it became more frequent in English towards the end of that century, following its adoption by the Times newspaper in Britain. (see the Oxford English Dictionary, 2nd edition).
The spelling czar originated with the Austrian diplomat Baron Sigismund von Herberstein, whose Rerum Moscoviticarum Commentarii (1549) (literally Notes on Muscovite Affairs) was the main source of knowledge of Russia in early modern western Europe. It is not found in any of the Slavic languages, but is the primary spelling adopted by Merriam-Webster's Collegiate Dictionary (11th Edition, 2003), with tsar offered only as a variant.
Correct pronunciation of tsar is /tsar'/ in SAMPA though many if not most English-speaking people pronounce it /zAr/.
Muscovite Princes and Grand Princes
Tsar was also the title of the rulers of Bulgaria in 893 - 1014, 1085 - 1396 and 1908 - 1946, and of Serbia in 1346 - 1371.
1 When Nicholas II abdicated in 1917 he abdicated not just on his own behalf but also on behalf of his teenage son, who was too ill to take up the throne. He named as his heir his own brother Michael. Michael initially accepted the throne and was proclaimed as Tsar Michael II. He subsequently declined it. Historians and lists of tsars differ as to whether to regard Michael or Nicholas II as the last tsar. Nicholas II was undoubtedly the last tsar to rule Russia and so was the last effective tsar. Michael, if he can be said to be tsar at all, exercised no governmental functions and merely reigned nominally for a short time before himself abdicating. Michael, like his brother Nicholas, was executed in 1918.