Tsar Alexei of
Russia was the son of Tsar
Mikhail Romanov. He came to the throne at the age of sixteen when his father died in
1645, and at that time was nicknamed "the Young
Monk." He was a religious man rather than a soldier, and he believed in the divine right of kings to control their lands completely (he was so shocked when the
English executed their king
Charles I in
1649 that he expelled all English merchants from Russia).
Despite his not being a soldier, under his reign the cities of Smolensk and Kiev were recaptured from Poland, and quite a lot of peasant revolts were put down; Alexei issued many laws that made much stricter rules for the serfs of Russia. He was also a mild aficionado of European culture (paving the way for his son Peter the Great to westernize Russia even more).
But Alexei's reign is remembered for two major things. The first is the Great Schism of the Russian Orthodox Church. The Patriarch Nikon, chief of the church in Russia, wanted to reform the church, making it closer to the Orthodox churches in other countries and especially in Greece and Constantinople, and Alexei gave up more power to the Patriarch than any tsar before or since. However, many Russians resisted these reforms; they had been taught that salvation rested on exact adherence to ritual, and even changing the number of fingers one used to make the sign of the cross could risk one's entry into heaven. These "Old Believers," led by a former friend of Nikon's, Avvakum, often fled settled areas and resettled on the steppes of the south or the other side of the Ural Mountains so that they could worship their way; some who were caught by soldiers the government used to enforce the church's policy were executed for their faith, or killed themselves before they could be imprisoned. Eventually (by 1658) Alexei and Nikon stopped seeing eye to eye, and eventually Nikon was stripped of his rank by an Alexei-summoned council of Orthodox Church leaders from other lands.
The other thing Alexei is remembered for are his children. With his first wife, Maria Miloslavsky, Alexei had thirteen children. The eight girls were all healthy and strong, but the boys were not -- three died before their father, and the other two were weak. The older, Fyodor, was named as heir, but he was not really expected to outlive his father. So when Maria died in 1669, Alexei remarried, a girl from a somewhat more Westernized household, Natalya Naryskin. They had only two children, but one was a very healthy son, Peter.
However, when Peter was not yet four, Alexei took part in the annual winter ceremony of the blessing of the Moscow River (performed on the holy day of Theophany or Epiphany). After standing out in the January air on the ice, Alexei was ill the next day, and died suddenly on February 8, 1676. His oldest son, the fifteen-year-old near-cripple, became Tsar Fyodor III.
Sources: Robert K. Massie's Peter the Great: His Life and World in addition to those listed under Monarchs of Russia.